My little boy is just over 3 months. He has worked out how to roll back to tummy and is determined to sleep on his tummy. He has not mastered rolling from tummy to back. We try to roll him back for sleep but he immediately rolls back even when asleep. He can hold his head up etc. How worried should we be about him sleeping on his tummy? Thanks!

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 his new environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Is he now in a cot? - not bassinet
Have you ceased wrapping/swaddling him?
Once rolling, babies need arms free & the space of a cot to roll well.
Ensure that the cot mattress is firm & flat & there is nothing soft or loose in his cot,
Always, continue to place him on his back for sleep.
**Increasing tummy time when awake is recommended to help build upper body strength so he can roll easily & well.

Have a look at the article linked above.
Let me know your responses to questions above.