What do you think about me wanting to have sex with my boyfriend on my period even though he doesn't?
Why doesn't he want to?
I think he thinks it's gross.
If he doesn't want to you should let it be. It's not a good idea to force the issue if he's really against it.
How do I convince him to do it if he's not into it?
You could gently explain why it would make you happy, try to negotiate something in exchange. But you should never demand he do something he's not comfortable with. You might have to just accept it.