What I missed in your post is what your son is feeling about it at school? Does he express loneliness? Does he wish other kids would play with him? Or perhaps he just wishes other kids shared his interest?
Is it safe to assume (posting here) that your son is gifted or highly gifted? If that answer is yes, then the next question is: are kids his age at school actually his peers other than by coincident of birth?
If he isn't lonely or concerned, then I wouldn't put that external burden on him. Friendly doesn't always mean extraverted, and introverts may very well need alone time at recess with their thoughts after a morning of being surrounded by other kids. If it is his concern and not a borrowed one and he is placed appropriately in school, then the above suggestions make sense.
If friendship in general is an issue, then I'd look for groups outside of school with mixed age kids that share his interests.
My DS8 through kindergarten and first primarily sought older kids and adults for conversation. With a grade skip into a gifted class he now gets some of his intellectual conversation out in that context, and, oddly, his best friend is a kid from his prior grade who has some overlapping interests with him. It's almost as if his first priority is getting his type of mental stimulation; sort of a "Maslow's Hierarchy of Gifted Kids Needs."
