C.M.
If he is doing well where he is, I would leave him there. You can do things in the evening too just to see his progress...reading, games, projects etc. You know him best. I would also recommend keeping him back from kindergarten. He has a late birthday AND he's a boy. I don't have boys but I have 3 girls...2 with November birthdays. #1 sent to kindergarten at 4. She could do all the work and was fine with the peers until about 8th grade. Hindsight is 20/20! #2 we lived in a different city and the cut off was Dec1. No peer issues later.
Keeping him in something that is goo AND covenient as well as economical for you is important too. Hope this helps!