4 year old son regressing at nursery school

My son has been going to nursery school since september 2010. He is an only child who turned 4 in May of this year. At first he was really excited and then went through what seemed like a fairly typical period of separation anxiety. After talking with teacher we decided to change schedule because they have a mandatory nap time that he hated (3 half days instead of 2 full days) and that helped. We've been cruising along with out incident until about a month ago. Now drop off is heartbraking. I have to hand him to the teacher and leave him crying for me. I try to make it short, sweet and positive. He has a picture of his family in his cubby in case he gets sad...that used to help at the beginning. Some days the crying stops right after I leave; other days it persists on and off through the 4 hours. We did change one of his half days to a full day. The teachers don't make him nap.  He has quiet time with books, toys or videos while the other kids nap. He starts stressing about it the night before and is genuinely worried. When we pick him up he's happy and interacting with the other kids. The teachers say he's polite, a good listener and a good helper. He's very bright, has a great sense of humor and is generally a pretty happy kid. I'm not sure what's going on or what to do. Any advice??

[margypops]

As he is the only one not napping(, it amazes me they put them to nap at all at 4 year old )..it could be he is alone and feels left out, I would say its something to do with the school do they have any attitude about him not napping , any disapproval he may pick up on ?