I wouldn't force my dog to play with dogs that are picking on him. Just my opinion. This could create a lot of fear and, in the future, fear aggression.
It's natural for an unsocialized pup to be scared in a social environment at first, though. My dog tried to escape the first day of daycare, but the next day he was playing with them -- plus I know and trust the staff. Increased GUIDED socialization will help, but if it keeps happening, I'd call it quits. The staff shouldn't be letting this happen either.
It's your job to protect your pup.
Warden - Sable GSD 4/23/2013
Blackjack - Black and tan GSD 4/23/11-6/1/2012