My boyfriend doesn't seem to get when girls are hitting on him

Me and my boyfriend are both juniors. We've been together since we were freshman and I love him a lot... It's just... When girls are blatantly hitting on him! Just today we were eating lunch together and this bitch from my class walks up to him and asked if he's going to home coming and he said yes with me and she says "Maybe we can go together! It's a little weird to take your SISTER to homecoming". He says "She's my girlfriend, but you can tag along if you want". Ughhhh. It just bothers me. My older sister said it's probably his Asperger's that he can't pick up when they're hitting on him. It goes right over his head and I'm sick of girls hitting on him in front of me!