My 6mo is placed on his back to sleep but rolls onto his tummy and prefers to tummy sleep for the remainder of his nap/night. At times, he moves his head to face flat down on the mattress, buried in his hands. I move his arms away and he will adjust to having his cheek on the mattress. But often reverts straight back to face sleeping and we go back and forth til he either wakes or gives up. He’s a very capable roller w/ good neck strength. How can I discourage this face sleeping? Is there anything medical I need to consider?

Most babies will be rolling over by 4- 5 months…...some earlier, some later.                             
This is normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy or side.
This is not something that needs to be stopped. It is important to make her environment as safe as possible.

**Has he been moved from bassinet to a cot now? & also ceased swaddling?
He needs the space of the cot & arms free to roll well.

Ensure that the cot mattress fits well & is firm & flat & there is nothing soft or loose in his cot, especially as he is face planting at times.
You can try to turn his face if you see him with face down, but often, like rolling to tummy, babies will just go back to their position of comfort.
**Increasing tummy time when awake is recommended to help build upper body strength so he can roll easily & well.
Always continue to place baby on back to sleep.

Red Nose has the following general information now that he is rolling.

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over