My son (7 mo) started flipping over to his belly after being laid to sleep on his back. he even puts his face down into his hands/mattress. is this okay since he hit his milestones? (sit-up, crawl, pull to standing.) if not, how do we stop it?

This is okay at this age. It is futile to attempt to stop the practice & the kid will have the physical power to move out of any position where his breathing is compromised. You did a great job starting him on his back during the early months when this mattered.

Continue to place him on his back for sleep until 12 months if age. The mattress should be firm, avoid other bedding near his head since this may compromise breathing. His rolling is appropriate for his age.