My 5 month old is rolling from back to tummy in his sleep and is sleeping on his tummy with his head to the side. He is unable to roll tummy to back as of yet but is it okay for him to stay sleeping on his tummy? Or do we need to roll him back onto his back?

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 the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Once baby is rolling, continue to place him on his back for sleep

Have you ceased swaddling/wrapping him?
Does he sleep in a cot?
Babies need arms free & the space of a cot to roll well.

What happens if you try to roll him back?
How does he go with tummy time when awake?
Does he hold his head well & reach for toys?

Have you ceased swaddling/wrapping him?
Babies need arms free & the space of a cot to roll well.