My almost 5 month old seems to want to sleep on his side. He is always put down on his back but will roll onto his side to sleep during the night, he is mostly on his stomach with his head to the side. I constantly check him through the night and roll him back but is there anything I should be doing to ensure his safety? It seems to be the main position that he self settles in but it makes me worry.

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 you ceased swaddling him & is he in a cot now?
Babies need their arms free & the space of a cot to roll well.
How does he go with tummy time when awake?
Is the cot mattress firm & flat?
What happens when you roll him back?

Always still place him on his back for sleep.

Have a read of the Red Nose information now that he is rolling . 
“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”
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over