My son is a week shy off 5 months old and in the last week has started rolling from his back to his tummy both awake and in his sleep. He hasn’t quite mastered tummy to back and has only done it once. In the last few days he has shown preference to sleep on his side and/or his stomach. I have tried rolling him back and he wakes every time and gets flustered with having been disturbed, leading to sleep disturbances for the both of us. Is he safe in this position? What are some things I could be doing? He is in a sleep sack with his arms free and his mattress is flat with nothing loose in the cot with him. Thank you in advance!

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 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

Have a look at the article linked above (if you haven’t already)
The important issues you seem to have already addressed:
in his cot (not bassinet)
arms free - no longer swaddled.
cot mattress firm & flat & nothing loose or soft in cot for now.
* Always place baby on back for sleep (even when they are rolling)

How does he go with tummy time when awake?
Increase tummy time when awake to help strengthen his upper body which assists with rolling.
Is he rolling both ways now?
Does he change his position during sleep time?