Uhh, in my last relationship I jacked off all the time. I have NO clue why your boyfriend feels so inclined, but here is partially my line of reasoning:
-Jacking off to internet porn is a ritual I've had since I was 14. It's comforting, and almost in a completely different category from sex. And goddammit, no one is going to take that from me
-Scheduling. Even though my girlfriend and I may have been equally horny, it typically happened at inconvenient times; she'd be horny when I didn't feel up for it/wanted to sleep, she'd usually already be asleep by the time I wanted it. Or at work. So we'd both indulge in the habit.
Not that suggestions from a failed relationship (mine) should have so much worth, but that's not the reason we broke up, so... Any number of explanations are really possible, from the innocent to the more nefarious or hurtful, so yeah, talk to the guy. In fact, it could be that communication issues are also the barrier to your sex life.