Its just part of the learning process. Pretty soon he'll be able to roll both ways. It just takes practice, and since he is probably on his back more, he learned that first. Not much you can do except maybe give more tummy time during the day so he has more chances to practice. Do it for short periods, but several times a day - thats what I'm doing. Mine is going through the same thing right now. I keep him in bed with me though - so at night when it happens I just roll him back... but most of the night he is on his side nursing anyways.