My 7month old son has started rolling on to his stomach in his sleep. He is doing this very often, sometimes he wakes up and will crawl out of it (he doesn't like rolling from his front to back very often). But a few times I have noticed he just keeps sleeping with his face straight into the cot mattress. I can't sleep not knowning if he will do this while I am sleeping, so have started co sleeping, which I really don't want to do long term. Will he grow out of this phase? Is there a way for me to keep him sleeping on his back in the cot?

Most babies will be rolling over by 4- 6 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.

Ensure that the mattress is firm & flat & there is nothing soft or loose in his cot.
It is important not to use bumpers (or other soft padding) in the cot, as he is learning to roll.

Most babies will just go back to the tummy, their position of comfort, if you try to roll them back.
Always continue to place baby on back to sleep.

If you are still wrapping him this need to be ceased - he needs his arms free.
Some babies do “face plant” at times. It is important that the mattress is firm, & there is nothing loose or soft in the cot.
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

Red Nose has a risk minimization approach to co-sleeping with information here:
https://rednose.org.au/article/is-it-safe-to-sleep-with-my-baby