Hiya
i was lucky with mine, she has a very outgoing personallity and took to nursery straight away, she loved being around other kids, tho she had the occasional cry in the morning when i dropped her off, you know the pulling of the leg please dont go mummy!! god it breaks your heart, but i called up as soon as i got to work and they said she was fine and happy playing!
all kids are different but you need to be persistant, they will get used to it, some sooner then others, but it will do nothing but good for your child, they learn so much from other children from a young age, its brilliant social and interacitve skills, just hang in there, you will find soon your son will love playing with the other kids and maybe even moan when you pick him up!! sometimes my daughter used to day " mummy, please can you come back later to pick me up and having lots of fun with my friends" lol
good luck xx