S.H.
My son was the exact same way, and just around this time he started doing the same. I tried reversing it by putting him down on his back to try and teach him to sleep the regular way, but he would wake up in 20 min every time. The thing is now that he has discovered rolling, they are more natually doing it in their sleep, however they don't know how to go back. I like the idea another lady had of doing "back time" instead of tummy time to help him learn how to roll over the opposite way, I also did this and I think it helped. Unfortunately I had to let my son cry it out a little because nothing I did magically worked, I also tried putting a blanket to position him too, but he would still roll over, I was out of ideas. Luckily it didn't last long, I think after less than a week he stopped fussing and started putting himself back to sleep and then it wasn't an issue anymore. He still to this day prefers his stomach, he's almost 2 now. It's a phase and it will pass, just try and be patient and get him comfortable with his back.