my baby can roll back to belly but not belly to back. for the last few nights he has rolled into his belly in his sleep and since he cannot roll onto his back we have been rolling him every 15 minutes and i’m exhausted. i always place him on his back. do we have to keep rolling him back until he learns belly to back or can we leave him on his belly if he rolls there from his back throughout the night?

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
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

Have you now ceased swaddling/wrapping him?
Have you moved him to his cot from bassinet?
Babies need arms free & the space of a cot to roll well & safely.
Ensure mattress is firm & flat & nothing soft or loose in his cot.
Continue to always place your baby on back for sleep.

How does he go with tummy time when awake?
Increasing tummy time when awake is important to build upper body strength to roll well (both ways)
Can he hold his head well, look around & reach for toys?

No parents are expected to sit up all night watching baby. This is not achievable.
Let me know your responses & I can provide a more specific response.