Perfectly fine and normal.
They often find it more secure and comforting to sleep on their bellies, and if he can roll into and out of the position, or at least has good head control, then I wouldn't worry. A nice flat and non-quilted surface and a clear space in the cot and then there isn't much you can do to prevent it, and if he is comfy - and safe - then don't try.
I personally would take him out of the sleeping bag and start dressing him warmer and using a blanket. Just me, but both of my girls (who have been tummy sleepers since birth) found the sleeping bag VERY restrictive to their movements when rolling in the cot, and TBH I felt the sleeping bag itself was more of a safety concern than the tummy sleeping itself, itms.

Anyway, totally normal behaviour, and if you are keeping the cot clear and he has the ability to move his own head, then he should be fine.