slumber party with older boys

My 9 yo DS is going to a slumber party at his friends tonight with boys that are 11 yo. My kid is kinda on the small side and I am afraid that others will pick on him. Plus getting exposed to older boy influences. I thought about not letting him go but those are my fears and he has been friends with this boy since he was a baby. What can I do to ease my fears and talk to him about making the right choices? Yikes. Not ready for kid to grow up...