My 5 month old boy has just started rolling onto his tummy about 2 weeks ago but he cannot roll back onto his back yet. When we place him into his cot to sleep in a sleeping bag with his arms out he rolls onto his side and sometimes onto his belly. We continue to roll him onto his back but he keeps going to his side. I am terrified of him doing this as I know the safest position to sleep baby is on his back but what can I do to stop him doing this? I can't sleep at night as I worry he is going to roll onto his tummy and have his face down into the mattress.

What you are describing is normal development.
Most babies will be rolling over by 5 months…....some earlier, some later.

This is very normal as babies grow & become stronger many babies then prefer to sleep on their tummy, & often don’t change from that position.
This is not something that needs to be stopped, but it is important to make the environment as safe as possible for this to occur.

Always place baby on their back to sleep.
Ensure cot mattress is firm, well-fitting & flat.
Ensure there is nothing loose or soft in the cot.
Use a sleeping bag now that has appropriate size neck & arm holes.
It may also be helpful to increase his tummy time when awake if you are concerned about his ability to roll.

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.com.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position