My 5 month old has started to want to sleep on his belly. As soon as I put him in his crib on his back, he then flips. I have also tried to put his face to the side however, he seems to want to bury his face into the cot mattress. This is making me extremely anxious. We do have a firm cot mattress with no blankets and nothing else in his cot, but I’m worried that he will not get enough air when in this position. What would you recommend in this situation? Thank you so much.

Most babies will be rolling over by 4- 5 months.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Once baby is rolling, continue to place him on his back for sleep
Is he rolling both ways?

Have you ceased swaddling/wrapping him?
Babies need arms free & the space of a cot to roll well.

How does he go with tummy time when awake?
Does he hold his head well & reach for toys?