Hi. My 10 month old sleeps and rolls onto his tummy to sleep and literally has his face straight down on the mattress. Do I need to keep rolling him back (he wakes straight up) or is it safe to leave him as he can roll

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

Ensure that the cot mattress is firm & flat & there is nothing soft or loose in her 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.
As you have found, there is not much to be achieved by trying to roll him back from his tummy.
Always continue to place baby on back to sleep.

Red Nose has the following general information 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.
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