My 5 month old can roll from back to tummy but not back again, and now likes to roll to sleep on his tummy in the cot. On occasion I have found him with his face down and had to flip him, im worried that I need to constantly watch him overnight now until he can roll back and/or consistently turn his head to the side??

Most babies will be rolling over by 4- 5 months…...some earlier, some later.                             
This is 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 his environment as safe as possible.

It is important that you have moved him to his cot. Have you stopped swaddling/wrapping him?
He needs his arms free & the space of a cot to roll well.
How does he go with tummy time when awake? - this also helps to increase upper body strength to roll make rolling both ways.

Ensure that the cot mattress fits well & is firm & flat & there is nothing soft or loose in his cot, especially as he is face planting at times.
You can try to turn his face if you see him with face down, but often, like rolling to tummy, babies will just go back to their position of comfort.
Does he wake when you roll him back or turn his face to the side?
**Increasing tummy time when awake is recommended to help build upper body strength so he can roll easily & well.

Always continue to place baby on back for sleep.

Have a read of the Red Nose information (link below) 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.

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