Like others, I second the "if it ain't broke, don't fix it" mentality. If he seems happy, is making friends, etc., keep him where he is. Maybe talk with the teachers about what you are seeing and encourage them to give him some additional challenge.
Only speaking for my child, we had some difficulty with the less structured "play-based" times at his former preschool. We think DS has some minor sensory-integration issues, and the "chaos" of the unstructured times was too much for him. (This is by no means a condemnation of play-based schools...just a note that the way it worked at one particular school was not a good fit for our son.)
Good luck!
Kathie
