My 6 week old baby is able to reposition himself onto his side from his back when he is in his sleep sack/swaddle. This seems too early to be really rolling, he also sleeps far better with his arms wrapped due to the startle reflex. Do I need to release his arms now?

Here is Red Nose information about when a baby is rolling.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

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.

Have you tried to gently roll him back when you see him on his side?
What does he do?
Is he in a bassinet or cot?

Recommendations once baby is rolling is to stop swaddling & move to a cot.
Babies need arms free & space of a cot to roll well.

How does he go with tummy time when awake?