My son is in Kindergarten this year, and I really feel for you. These decisions feel so monumental that it's really hard when they get complex. (And when well meaning people, meaning my dad, tell you to relax. Don't they get how life changing these decisions are for us?)
A couple of things to consider. My son has a July birthday, so the decision facing us was whether to put him in the regular kindergarten or the 2-year one. (We are so lucky that our school district has this option). We chose the 2-year, because I heard from so many moms of boys with summer birthdays that it made all the difference in the world to let them begin kindergarten at age 6. They were more mature, could sit still better and follow directions better and just had an overall better experience. Mom's of older boys told me that they could tell in later grades that being the oldest was such a good thing for them. No one regretted having their son wait, but there were some moms who said they wished they had. There were some that sent them at age 5 and said they had no regrets.
So, one thing to consider is whether or not placing him in kindergarten next fall would be a good idea.
Right now he's attending "the best school." Those can be dangerous words. This school might get great reviews and produce great scores, but that doesn't make it the best school for every child. It also can keep you for considering what the next best school has to offer. Often times the next best school is pretty darn good. Also, you really do want to find the school that is the best fit for your son, and it's ok if this school is not it.
Of course you're not sure, this really is a big decision. Maybe you could meet with your son's teacher, talk to her about your concerns and let her know that you are wondering if you made the right decision in choosing this environment. Maybe she will tell you that she really does believe it's the right school for him and that he does have ADHD. She might tell you that she hadn't considered that as a possibility and would give that some serious thought and observe him more with that in mind. Who knows, but it's a place to start.
I'm sorry I wrote so much. I just wanted to give you a couple of things to reflect on. Good luck!!!!!! I really do know how overwhelming this is for you.