Reddit reviews The Body Book For Boys

Get him a book and discuss it together. Note the second half of that sentence - this doesn't get you off the hook from talking to him, but gives you a place to start and some idea of age-appropriate language. It's Perfectly Normal is widely recognized as one of the best sex ed books out there (and does cover masturbation); if you want something for boys specifically, you could try the Boy's Guide to Becoming a Teen or The Body Book for Boys (not completely sure if they both cover masturbation so you might want to read some reviews first).

Having said that, I'm wondering why Mom wants you to be the one to have this conversation in the first place? Do you have a relationship with him such that he would feel comfortable talking about such personal stuff with you? Otherwise I would probably suggest that she bring it up with him first, and then you can mention that if he has any questions he'd feel more comfortable asking another guy, that you'd be happy to chat with him too.