My son is 5 days, started rolling to his R side while sleeping In the bassinet, we then move him to his back but he immediately move to his side again. He doesn’t sleep in a swaddle. We’re scared because he’s so young and might no be able to roll back very well. What can I do to prevent him rolling? Thank you.

Most babies do not roll to tummy intentionally before 3 months; however, some babies do roll, especially to the side, at a very early age. This is not unusual.

Some babies do not respond well to wrapping/swaddling & prefer to have their arms free.         
Some babies like to touch their face or suck fingers when settling.
Continue to keep him unswaddled with arms free.

Are you using blankets?
It is recommended to move him to a cot rather than a bassinet.

Have you commenced tummy time yet when awake? You can start right from birth.
https://rednose.org.au/article/tummy-time-is-good-for-baby-because
Always place baby on the back for sleep…. including babies who are rolling

You are not expected to wake during the night to keep checking on him.
However, there is nothing wrong with rolling him back if you see him on his side.

Red Nose does not recommend the use of sleep positioners.
Red Nose does not recommend positional products such as anti-roll devices and items that fasten a baby into a sleeping position.
Products that restrict the movement of a baby or a baby’s head should not be used.
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position