I always place my 5 month old on his back to sleep in his cot, however he will immediately roll over onto this tummy. He seems much more settled and comfortable on his tummy and will wake and get fussy when I roll him back over onto his back. He cannot yet roll from his tummy to his back on his own. Is it safe for me to let him roll over and sleep on his tummy if this is his preference?

Red Nose recommendations are to “always place baby on back for sleep” not baby must always sleep on their back.
https://rednose.org.au/article/red-nose-safe-sleeping-recommendations

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
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 the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Have you now ceased swaddling/wrapping him?
Babies need arms free & the space of a cot to roll well & safely.
Ensure mattress is firm & flat & nothing soft or loose in his cot.
Continue to always place your baby on back for sleep.

How does he go with tummy time when awake?
Increasing tummy time when awake is important to build upper body strength to roll well (both ways)

As he not yet able to roll both ways we would encourage you to gently roll him back, particularly when you see him on his tummy, but you are not expected to sit up all night watching him.
If, however,  he then rolls straight back to his tummy, you are not really achieving anything.
There is nothing you can do to stop him from rolling.
Ensure all other guidelines are in place & increase tummy time when awake.
The safe environment continues to remain important.