You're right, you cannot check on him all night long, and now that he knows how to roll over, he'll move how he wants when sleeping, regardless of how you wish him to remain positioned! Take everything out of his crib including blankets and toys, and just put him in some warm pjs at night so he doesn't need a blanket and let him be. As long as the crib sheet is nice and tight over the mattress and there isn't anything extraneous in his crib, I wouldn't worry about SIDS. He is now at an age where he can consciously move his body in a big way, and there is no stopping that - most children that die of SIDS suffocate because of someTHING that gets in the way of their nose and mouth (stuffed animals, blankets, sleeping in an adult's bed) not just from being able to roll onto their tummies. Most babies actually prefer to sleep on their tummies, so let him be and just make sure his crib is clear of stuff for your piece of mind.