My son is 5 months old today. He is unswaddled in a sleeping bag with free arms. For the last week he has been rolling onto his side and tummy when sleeping. For the last 2 days he can only get to sleep on his side which is completely new. I try and try to get him to settle on his back but he doesn’t want a bar of it and gets quite upset. I watch him on the monitor during the day but I’m concerned about his sleeping on his side and tummy during the night. I try and roll him on his back once he is asleep but within a minute he is back on his side.

Most babies will be rolling over by 4- 6 months. This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy or side.
This is not something that needs to be stopped. It is important to make her new environment as safe as possible.

If he is still in a bassinet, it is important to move him to a cot. He needs the space to roll safely.
Also ensure that the mattress is firm & flat & there is nothing soft or loose in the cot.
Continue to always place baby on back to sleep.
Increasing tummy time when awake may also helpful.

Red Nose has the following information now that your baby is starting to roll.

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over