My baby is 5 months old and he has already started rolling. When sleeping in the cot, he can’t seem to fall asleep on his back. But when he is on his tummy, he falls asleep. I tried turning him around but he woke up crying very badly but fell back asleep on his tummy. What should I do?

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

Once baby is rolling, continue to place him on his back for sleep & let him find his position of comfort if he is rolling well both ways.
How does he go with tummy time when awake?
Does he hold his head well & reach for toys?
Is he able to roll both ways?
Have you ceased swaddling/wrapping him?
Babies need arms free & the space of a cot to roll well.