To be honest, I wouldn't start thinking about ADHD or something labeled alike. I wouldn't take it to a psychologist or any type of counseling YET. I would start at home. Most of the time, children behave and act according to their environment or according to what is happening inside their families, and sometimes we, moms, are able to deal with it, even with lots, lots of efforts and dedication and patience..but we can do it! It is worth to give it a try before taking the easy step to treatments for psychological and behavioral problems (drugs!.
Obviously, he is a smart and sensitive kid and it would be very good if, first of all, you talk to him about his feelings without punishing or criticizing him. You will be able to drag something from it. Let him talk, give him some confidence and make him feel like he is loved and you are able to understand what he has to say to you. After that, try to make some "Mommy and me time" with him and do fun things where he can vent a little bit more (running, jumping, swimming or just racing with mom) Read with him stories with animals about obedience, respect, friendship, kindness something like that and keep talking about his feelings, why he is doing that or this. Make him to take a short nap everyday and do not let him to go to bed past 9 o'clock. Laugh with him and talk to him about being kind with others and not hurt their feelings; that way he will have more friends and will have more fun.Do things to help him to relax, listen music and dance with his little sister. Do everything I suggest, every single day. It is exhausting but I think it is worth it. Just try and if you are upset about things try not to show it, he will see it and will learn from it!.
Of course, I think discipline should be there, if he is having outburst at home or at school, just take some privilege or toy away (TV, favorite food, etc) but let him know directly the reason why you are doing so. Be firm and be credible. I am telling you this because it has happened to me and it worked all I said to you now. Also, make a plan with his teacher, she will very able to help you more and give you some ideas. Work together.
After that........you can go and talk to your pediatrician and then a psychologist...
Good luck and I really hope you find a solution. Let me know please..