have you had a word with his teacher, sometimes we parents worry because we think the kid is scared/shy at school, but quite often once mum and dad have left, the kid has a great time playing with the other kids.
if he is feeling lost and lonely, perhaps he could be paired up with a similair child, like buddie,s to play together and give confidence, or maybe u make friends with oter parents and invite a child to come for tea, one at a time may better than loads of kids all at same time. he prob just needs time and confidence.
good luck.