How did he "find out." Did someone tell him? Poor little fella.
It's not that he's no fun. It's just that what he thinks is fun, may not be what the other kid thinks is fun.
My advice would be to involve him in something. By the time you get to highschool, you can join social clubs, athletics, band, choir, all sorts of stuff. There isn't as much at school for 7 yr olds, but help him find people like him if you can. Maybe science club or if you want him to be really cool - put him in karate. My kids are in a bunch of sports at the ymca, so, they make a lot of friends that way. And it helps thier socialization skills.
I hate to even suggest this, but it works. You could buy him some super cool new fancy shoes or backpack or video game. The other little guys would probably pay attention to him long enought to find out he's actually pretty cool and if nothing else, it would give him a boost of self confidance.