My 7 month old was in a hip dysplasia brace until he was 6 months, that prevented him from rolling. Since having it off he has started sleeping on his tummy. He can roll from back to tummy and tummy to back and is strong. My concern is that he seems to sleep face down at times in the night. I'm wondering how do they breath in this position. I'm much less concerned when his face is to the side. Should I be repositioning him? I've been watching the monitor like a hawk

Good news that your baby is out of his hip dysplasia brace.
Most babies will be rolling over by 4- 6 months. This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy.
This is not something that needs to be stopped. It is important to make his new environment as safe as possible.

Ensure that the mattress is firm & flat & there is nothing soft or loose in his cot, especially as he is face planting at times.
Some babies just seem to face plant, but it mostly is brief. This is when it is particularly important that the mattress is firm, & there is nothing loose or soft in the cot.
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 the position of comfort.
Always place baby on back to sleep.
Increase tummy time when awake.

Red Nose has the following information now that your baby is starting to roll.

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