Just keep doing your best to be a good mother. It might not hurt to really look at your relationship with him, but I'm sure you have done that already. Lets face it, mothers and fathers are different so we treat our kids differently. The important thing is to have the same basic principles and to agree on the major stuff.
You and your son can still go for family counseling. All you can do is deal with the things you can control and hope your SO will see your relationship with your son changing for the better and decide to give it a try. If he doesn't want to go, then even if you get him there it won't do any good!
An ADHD support group probably has many people there who's kids are co-morbid with something else. I would go and decide for myself whether it was the right place to be.
You're a terrific, loving mom! Other people may find it easy to criticize, but if they were in your shoes, they would change a lot of their opinions! I did a total turn around in my beliefs about ADHD when I was blessed with my son! Sometimes you just have to mark other people's opinions down as ignorant and keep on keeping on! 
It is very diffucult to convice members of your family that your child has such a problem. I have found that the men in my family don't want to accept the fact that my son is not always able to control his behavior. "He's doing it on purpose," "He knows exactly what he's doing," You let him get away with too much and that is why he is the way he is." I feel that I'm not only working with a challenging child, but also with a challenging family. If we could all work together it would be so much more helpful to my son. My boyfriend is not interested in family counseling, but I'm still working on it. I've explained that it wouldn't be about him or an attack on anyone, it would only be to work together for my son.
My son's father disappeared when I was six months pregnant, therefore; he has never seen his father and he is nine years old next month. I've been with my boyfriend for five. But you are right that it is not the same with another man that has not been with him right from the beginning. I'm just so frustrated. The day to day arguments with my son, defending him to my boyfriend and sometimes my parents.
I have finally found a support group in my area, but I was told that It my not be for my, because my son has other issues besides ADHD, that can be severe at times. I just keep on fighting for him and searching for help, and because I love him so much, I will never stop.
My daughter treats me different, I think partly since we are so much alike, but I think because I am the one who has been around forever. Her step dad has only been around for 3 years, and her real dad hasn't seen her for almost 4 years.
I am constantly being told that my son treats me different than his dad, because I am too easy on him, or too soft.Most children treat their parents differently. It may not be that you are too easy or your husband is too hard but you are each different. It is a good idea that both parents be consistant with AD/HD children.