Joesmom is right that it's not nessecarily part of adhd, but you're right to think that his impulsivity is causing him keep it up. I'm sure you've told him to stop, but kids with adhd have a lot harder time actually doing that than other kids.
What do you mean by "acting sexual"? By this age kids are becoming more aware of how their bodies work, how babies are made, and are beginning to understand that it is associated with pleasure- this can be confusing, perplexing, fascinating, and disgusting for them all at the same time. They are also bombarded with sexual images in magazines and on television.
It's pretty typical to see kids this age making gestures and talking about their genitals- my son is nine and he's just starting to pick up some of the slang terms for body parts and he likes to try them out on me- he appears to think I won't have heard them before. Another thing he does is anything that he runs across that is long and cylindrical (like a paper towel tube) becomes an, eerrrmmm, extention, so to speak, and he holds it in front of his crotch and jumps around the room and whooping loudly. Not exactly desirable behavior- and it's definetely worse if his meds have worn off. We handle it with warnings and then loss of marbles (see marble thread at top of page), and try not to make a big deal out if it as we don't want to reinforce the behavior with negative attention.
If you're talking about looking/touching kind of stuff then you definetely need to have a sit-down talk (I find building with legos while talking is helpful- gives him something to keep the hands busy and to look focus on if he's embarressed, but he's still hearing me and answering). Explain (probably not for the 1st time) that people's bodies are there own, the only people who get to look/touch there are him, doctor at check- up, and parents if there's a problem, and that it makes other people uncomfortable and embarressed when he does that- kids with adhd often fail to notice body language so he may not realize that his brother is uncomfortable with this. Then let him know that there will have to be consequences if it keeps happening because he knows that it's not appropriate.
Another that we do is have a sort or running conversation about this kind of stuff- my dh and I have decided to always take the opportunity when this stuff comes up tp open the door for questions because sometimes that's what it's about- my theory is that when my son has questions but doesn't know how to ask he sort of acts it out behaviorally. We always try to pretend like we're totally comfortable talking about it (even if we're not) because we always want out kids to come to us, not their misinformed friends when they need info on sex and reproduction. We were both raised by parents who didn't talk about sex (his not at all, but he has a brother 7 years older than him so he learned lots that way, and mine only told me don't have sex) so we decided to take the oppposite route, givent he various difficulties that lack of info cause for both of us at different points.
And I shall conclude my book by saying. . . sorry, maybe I should write a book on the subject. 
Jaderocks54...not a book at all-that was really helpful. His friends are starting to talk about all that & they are beginning a unit at school(5th grade!) so I know it is on his mind...not to mention some of the shows like Family Guy that he will sometimes sneak when we aren't aware! I like the marble idea-he has been pretty good with those types of systems & he definitely is not aware of the body lang. cues that others give.
Also, love the legos..we see a therapist who does that & it is night & day- maybe I have been too in his face trying to find out why he is doing it. I am going to try that with him. He does that silly stuff - but the touching also - it is really frustrating-but we will continue to work on it
Thanks again!
Tell him to stop it and it is NOT funny explain that it is not appropriate to make sexual jokes with/infront of an 8 year old . I would give him some consequences if he continues and then if he does not stop talk to your psychologist.
This is not a part of ADHD
My son was dx at 8-mild ADHD. He has some space boundary issues and can be spacy, etc - but as he is getting older, we notice he acts more "sexual" - Seems to be only directed at his 8yo brother & seems like he is doing it beacuse it really annoys his brother, but it seems like it may be impulsivity? He does it in a way where he comes off as it is funny & makes it like it is a joke...He is a pretty gentle & quiet kid-most of his acting out is with his brother-never with friends, so we are not sure what this is about. Is he trying to get his brother mad or is this something more serious? Has anyone heard of this as part of ADHD?
Thanks!
Jaderock made some really good points and suggestions. You need to remember that at age 10 your son is prepubescent. He is discoving the pleasures that his hormonal changes are causing.
It is very important that you acknowledge his feelings and also point out to him about the inapropriateness of his actions. Do not shame him or negate his feelings.Let him know that everyone experiences these feelings. Give him some positive ways he can deal with his feelings. At the same time, explain why he isn't allowed to act in certain wayss and in front of certain people.Then let him know what you find unacceptable and the consequences if he disregards your 'orders'. Any self stimulation belongs in the privacy of the bath or his room at night when alone.
Don't be surprised if ,when with his friends ,he does some rally weird stuff. I can remember my own boys comparing their sizes, seeing who could pee the farthest, etc. Boys are a different species. lol!