Ruby has rolled and slept on her tummy from around 4.5 months...we were so worried and spent hours turning her, checking, turning, checking....
Now we let her nap on her tumy, and when she goes down @ 7.30pm we leave her on her tum. At 11pm ish when we go to bed, we turn her to her back and tuck her in with a thin sheet so she stays through the night on her back but 9 times out of 10 by morning she has wriggled out of sheet and is on her tum again
Nursery turn her onto her back when she naps, they check her every 5 mins in her cot, and she is ALWAYS on her tum, they put her on her back, and by the time they check her again, she on her tum
She sleeps with her face resting on her hands so her nose and mouth are raised above matress, but she always wakes with little red hand imprints on her face, is soooo cute
I really wouldnt worry, as long as he is happy and sleeping let him find his own comfortable position