My son didn't start playing little league until rookies, so I can't tell you anything about tball.
Based on watching my son play rookies and farm, I would consider one thing that you didn't mention. Can your son stay focused on the game? Most of the boys on my son's rookies team could pay attention in the field. There was one boy who was building sand castles in the infield. This wasn't very safe and was really frustrating for the kids.
There are varying skill levels in rookies, so if your son isn't great yet, I wouldn't worry too much. If you think the coach does a great job and your son enjoys playing with him, I might have him play on that coach's team.