Transgender boyfriend?

Okay I really like this guy, and he likes me to, We're in 9th grade but before we decided to get into a serious relationship he came out to me as transgender, and explained his situation. I was preparing to ask him out when he sat me down and explained the whole story and how long he has been Trans... Well I talked to my mom about it and she thinks I shouldn't date him. She is kind of homophobic which bothers me a lot. I'm straight but I still like him and don't want to hurt his feelings... What should I do?  Oh and a second question, if we decide to date and it gets into a serious relationship, how would our sex life work?