I agree with BETHANN about getting an evaluation. Knowledge is power.
You can also work on a positive reinforcement behavior plan (check out the really long stickied thread about Ogram's marbel system) and see if that helps.
I wish I had known more when my youngest was first showing signs. I thought she was just very active like I was at that age; and stuborn like her dad. We could have saved some heartache. But things are good now, we have a handle on things.
Good luck with this journey, where ever it leads.
I feel your pain. I have to admit when I read your post, a short part of me said "nah, that sounds pretty normal at the age" but then I have to remember that thats what everyone said when I would describe my DS until they actually saw what I was talking about. I would go get an evaluation as well. We finally had my son evaluated about a month ago (he is 4) and as much as it broke my heart to find out the very thing I was suspecting, I was glad to finally have some answers. WE have not put our son on meds as of yet, but hubby and I both agree that maybe around 7ish we might if this is still a problem.
Your comment of the swimming class brings back a memory for me too. It was also a swimming class situation and thank goodness the teacher was patient with him but I was just waiting at any moment for her to just scream at him because he was so impulsive and could not listen to a single direction. I want to so badly put him in a activity, but he really cannot follow directions unless it's really one on one.
I came across this board because I am searching for answers on how to best help my son. He is in kindergarten, keeping up with his school work but always on the teachers radar. At his recent report card review, his academics were fine but the behaviour portion was not satisfactory. "He doesnt keep hands to himself, doesnt respect others property, does not follow direction." At swimming lessons this summer he was never attentive to the teacher, always popping in and out of the water, bouncing up and down, picking on the other kids, never sitting still. He acts that way when he is around others and does it with me only when he is tired or overstimulated/excited. When he gets to this point, there is no reasoning with him, i have to remove him from the situation (usually i put him in his bed long enough to calm down). Now that he is older and teachers have to deal with his behaviour my concern is that they will not have the patience and label him a problem. Is is already happening and they cant understand why sometimes they cant reason with him or get him to stop his over the top behaviour. To make matters more challenging he is the height of an eight year old so everyone he comes in contact with expects him to act like an eight year old because of his size. Any advice would be helpful, I am beginning my search for enlightenment.You description of your son is my son at that age. Mine was diagnosed just shy of 5. I went the denial road, but regret it.
My son is adhhhhhhhhhhhhhhhhhhhhd, very impulsive and hyper. The teacher told me that he truly didn't even knew he did something until it was over and someone said something. He was in motion always, motor driven.
He is now 11, been on meds since he was almost 7 and doing fine. He also was always one of the taller kids in class, looked older, but sure didn't act it. DS was also the youngest age wise in class. This sure didn't help matters either. He is a June birthday.
The kids in school were cruel from kindergarten on, he got labeled and made fun of. It is still there, but he is strong. He is a better person because he has told me he will never make anyone feel the way that they made him feel. He also understands adhd.
He had no playdates after 1st grade so I enrolled him in activities where he could meet new kids and have a social life.
We ended up starting meds at the end of 1st grade with the help of his teacher, she was wonderful. We tried behavior mod. with her, but that was wrong, he couldn't help himself and was not fair to him to lose his recess.
I would have your son evaluated then you will know for sure. I truly know how hard it is, as well as heart breaking sometimes. This way you can start before he gets labeled.
Come here to the boards, it truly helps!!
Keep us posted!! 
Thank you for the wonderful advice. I am thankful that he only exhibits this out of control behaviour every so often, but when it happens it is usually a biggie. Mostly when he is overstimulated. When he was younger, he was constantly on the move and loud. I have learned not to react when he is like this because its like feeding fire. I am trying to look forward and prepare for whatever comes our way. If he begins to struggle more at school I want us to be a prepared for the next step.
I also was afraid to put him into lessons and group activities, but he has actually improved quite a bit because he is learning some self control. He played Tball and soccer, and did well at both. No matter what, I tell him everyday how awesome he is and that we are so proud of him.
Dear PJRW-- Your son has a BIG advantage...you and your husband! You sound both caring and insightful. And bright. If you and/or your husband are bright/gifted, likely that your son is too. And that could be contributing to his being prone to getting over stimulated and over excited.
[QUOTE=PJRW] ...No matter what, I tell him everyday how awesome he is and that we are so proud of him... [/QUOTE] Keep it up. Keep it up. Keep it up!!! The biggest threat to your son's ultimate success is not the criticizm by others, but his own self criticizm. There's only one defense for that, and it's in the self confidence and understanding you, his parents, establish in him now. It will prevent the seeds of self-criticizm from rooting within him. Once established, his self confidence will shed all the criticizms hurled his way, like so many weed seeds washed off a concrete patio by a spring rainshower.
My son is easily overstimulated. THey say it is part of his sensory integration, but it sure fits with his ADHHHHHHHHHHHHHHHHHHHHHHHD.
We have him on guanfacine in addition to the concerta. he only takes 1 mg in the AM. If we feel that a sport practice might be too much for him, he is prescibed to take another one in the afternoon/pm if ncessary.
Just a thought, some have their children ONLY on guanfacine/tenex - same thing.

I totally agree with John D's post, keep up the positive reinforcement. You may be the only giving it to him, especially if he gets in trouble at school, etc. That is so true about his self esteem. He will want to do better, only
I do the same with my son, and I know he cherishes it! They need all the encouragement they can get
Great advice John D.!! 
PJRW: one more thing.... I was teaching children how to swim some years ago--first time I taught and I had three classes in a row every saturday morning for 8 weeks. Little tykes like your son. This was before I found out I had adhd. One boy, Tyler, stood out because he behaved just like your son. Seeing him in my class the first day, I was not happy! He was a whirling dervish! I thought he'd be nothing but trouble. He wasn't though. Sure, he hardly ever looked at me when I was talking to the class as a group, and even when working with him one-on-one he'd only make eye contact for a nano second. But he loved the water, and despite his appearnace to the contrary, he was paying attention--in his own way. He progressed faster than many of the other kids in his group. By the end of that 8 week session, of all the kids I had, he had taught me a valuable and lasting lesson about differences in learning styles and the need to adapt to them. I think he and I "clicked" because, subconsciously, I saw in him the boy I used to be, and I knew that if made to hold still, his light for learning would be extinguished.