I think the issues you've expressed should be addressed. If your pediatrician won't deal with the issue, then I think you need to either a) get new pediatrician or at least a second medical opinion or b) contact the school for an evaluation.
However, about kindergarten, I think you need to send your son. The issues you are addressing should not affect his education. In fact, an educational environment may finally get him the help he needs!
You don't mention if he's in school now? If so, talk to those teachers for help. If not, then maybe a little "peer pressure" at school will help him to get onboard.
I am not at all suggesting that you caused this! I have seen other boys who consistently show potty training issues at home, but miraculously at day care or preschool do just fine. It appears that they know what they can get away with at home and the teachers simply have too many other kids that they don't "baby" them.
Have you tried treating him like he's in school? As in, stick to a schedule, take frequent breaks for potty, don't allow the undressing, have him sit a the table to do "homework" while the bigger kids do theirs, etc.? I realize this may seem like "too much" for him, but how do you know if you don't try?
I agree that kindergarten is still a ways off. You have a lot of time to work with him on "acting like a big boy". Until you find out if he can be evaluated by the school, I'd try to start working with him on your own to listen to a story, if he gets up, tell him, we're not done yet, please sit back down. Same thing for "homework" - here I'm just talking coloring, pasting, using scissors, practicing letters, etc. We have "kindergarten" workbooks from LakeShore Learning. My kids often "do homework" while I cook dinner. I think it helps them to work on their own, practice their fine motor skills and get better at "school" type work (beyond coloring!).
As for the potty training, part of that may be that he needs reminders on when to take breaks. A schedule may help with that since they do take breaks at school. Also, with summer coming up, you can work with him more since the clothing isn't so restrictive. I know our kindergarten teachers ask that kids are only sent in clothes they can get up/down themselves as they DO NOT DO BATHROOM HELP. So try to keep your son in elastic pants, simple t shirts (so the bulk won't get in the way), etc.
Finally, our school does have kindergarten prescreening. I've learned they do this to a) determine where every child is starting at with basic social, verbal, reading, writing and listening skills and b) to then place children in classes so that the "high" performers are intermixed with the "low" performers. (I'm sure they don't use those terms, but you get the idea. They don't want a teacher overwhelmed with too many kids that need a lot of extra help.)
I think you are absolutely right to get your child evaluated now. However I also think that kindergarten is an "age" based educational move for kids. I think you should send him. It's the teachers/school district's job to help you and your son determine if he has special needs and then help him along, both socially and academically with other kids.
You already know he's a smart kid. Sending him to school with get him the social help he needs.