My 15 year old son hangs around a bunch of 10 yr olds and I'm afraid he likes one of the girls?

Anna, you are the parent. You have a 15 y/o that is friends with kids much younger than he is. There are a ton of things that he could be involved with besides 10 y/o.

If he has interests, such as chess, reading, etc., start looking for clubs either in your city or at his school. If he plays sports or wants to, look into that. He needs to be busy at home doing chores as well, so he is contributing to the family. Find some volunteer work for him to do.

It's summer and there are generally a ton of things going on. If he is telling you that kids make fun of him, try and find out why. He could be socially awkward and needs practice talking with others.

Continuing this friendship with kids much younger than he is will not lead to good things. As his parent, step in and end this and help him discover other ways to find friends.

The potential risks involved here FAR outweigh any potential benefits.

And I say "potential benefits" only on a theoretical basis since I can't think of any that might exist.

And if his not having any friends is a concern to you, what could a 15 yr old boy and a 10 year old girl possibly have in common (appropriately) to base a friendship on.

Your son needs to start making friends closer to his own age.  You need to do two things---1) tell him that and ---2) help him do that.

A 15 year old is still a child.  He's obviously making bad choices.  That's why children need adult supervision to help them make the right choices.  There's something wrong if a 15 year old is interested in 10 year olds.  You need to intervene and get some help with this if you can't handle it.