Feeling bad for step son

My step son is 15 and so shy and has bad anxiety. He's a nice kid, just has trouble communicating. He gets in trouble in school, but for silly things like not participating in class discussions. Also one thing he gets in trouble for is ditching class. Well to classes specifically. Gym and math. He gets bullied a lot in his math class and he isn't good at math. He has bit of a stutter and they bully him for that. In gym he isn't very athletic and doesn't like his body that much so he hates wearing shorts. The town got together at a watermelon festival and we were all eating watermelon. My step son was reading Diary of A Wimpy kid. He has most of those books and he loves them. Some girls come over to him, I'm on watching everything while helping with the ticket raffle. She talks to hi about Diary of a Wimpy kid and he seems to enjoy the conversation. She says "My siblings love those books... They're 8 and 10". They laugh and take it from him and I yell " Hey! Cut it out!" They throw the book on the ground and my step son hides it in the car obviously embarrassed. I keep telling my husband he has such low self esteem but my husbands just like "Eh. He'll be fine". He doesn't have no one friend....