My 3 and a half month old almost refuses to sleep on his back. He initially falls asleep on his back but after his first night feed as soon as i put him to bed he instantly rolls to sleep on his belly. He cannot roll from belly to back yet so i dont know what to do. I am losing sleep because i try to keep him on his back for at least 15 minutes but in the end he will only sleep on his belly.

Red Nose recommendations are to “always place baby on back for sleep” not baby must always sleep on their back.

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

Are you still wrapping him? Is he in a cot or still in bassinet?
Now that he is rolling, it is important to move him to a cot & to stop swaddling/wrapping.
Babies need the space of a cot & “arms free” to move well
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)
Is he rolling from tummy to back during tummy time, holding his head well & reaching for toys?