Boyfriend wakes me up at night?

It seems like if I don't sleep with my boyfriend before I go to bed he spends half the night grabbing me with his hands and poking me with his thing on my backside. I yell at him in my sleep but then like an hour later he's back at it again. He's not a mean or aggressive guy. I've had other guys do this but not on this level. What should I do? Guys - why is he doing this?