I think you have to look out for children with common interests and sort out social arrangements for them at that age. If he's not played much with a child before, he may struggle to judge what sort of games/activities that he also enjoys a lot are most likely to work, so you may need to be a little more hands on in guiding things. In a small school it can be hard as there's fewer to choose from, but don't rule out the girls - they are often more mature and sometimes cross-gender friendships can work very well at primary age.
I'm not saying you should discourage playing with older children if that's where he's happy, but be aware it can sometimes go awry if the older kids decide they don't want a youngster around or want to do stuff that's perhaps not so appropriate for someone 2 years younger, more that you should actively encourage peer friendships if possible.