As someone who didn't sleep train, I would generally say leave him. He will learn to get comfy on his belly until he learns to roll back over the other way. I tried flipping mine each time she rolled onto her belly and got upset, but then it just started happening more frequently. When I finally gave up, she started settling on her belly (and now sleeps that way every single night at 18mo). You will very likely go through the same thing again when learning to crawl, stand, walk.

The best thing is to make sure they've got plenty of opportunities to practice new skills during the day - so lots of tummy time right now. You can encourage rolling onto the back by helping position baby a little bit (I found that helping mine get both arms up under her chest helped, then when she was able to push all the way up well, showing her that laying one arm down flat on the ground while pushing up with the other arm helped give the momentum to roll back over), but they'll figure it out when they're ready! Putting toys off to the side just out of reach helped with motivation, too.