C.S.
I don't know if it's the same every where, but in WI it's a regulation that kids at DC or preschool must nap/rest until they are 4.
Kids don't have to fall asleep and often don't. My son is 3 1/2 and now it's more a 'rest' then a nap. As long as the kids are quiet some sleep, some don't.
Be patient-with the seasons changing and it getting dark earlier plus just the extra energy he'll expend at preschool he'll probably adjust to an earlier bedtime fairly quickly, especially if he doesn't actually nap at 'rest time".