My 5 month old can only roll onto his stomach and not the other way, at night I find him sleeping on his stomach. Do I need to keep waking him to flip until he can roll both ways ?

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 tummy.
This is not something that needs to be stopped. It is important to make his new environment as safe as possible.

Ensure that the cot mattress is firm & flat & there is nothing soft or loose in his cot.
How does he go with tummy time?
Increasing tummy time when awake.
Always continue to place baby on back to sleep.

I assume his arms are free in a sleep bag & is no longer swaddled/wrapped - he needs his arms free.
Is he now sleeping in a cot?

Does he wake up if you try to move him back from his tummy?
You are not expected to be awake all night to roll him back, but it is important that his environment is safe.

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