Boyfriend insecure because he cums too quick

I've been with my boyfriend 6 months and we haven't had sex even though I wanted to. He opened up to me and said he's embarrassed because he cums way too quickly and he's tried everything to learn to last longer and every girl he's ever had sex with (2) has gotten annoyed and mocked him for cunming to quickly and then be would lose his erection. Honestly I don't understand why people mock guys for cumming fast. If they just cum and they're done and don't get you to finish I can understand being pissed but if they just cum quickly I don't understand making fun of them. How can I make him feel better about sex.