Hello, we have a 5 1/2 month old that likes to fall asleep on his side whilst we are tapping him to fall asleep but then when we go to turn him on his back he wakes up again and is quite upset.. We feel that he prefers sleeping on his side. Why is it a SIDS risk to leave babies sleep on their side. Is it an absolute must to turn them on their back and until what age please?

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?
Have you moved him to his cot from bassinet?
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)

Is he able to roll both ways? If not, then 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 he continues to resist, there is not much to achieve by rolling him back.

Can he hold his head well, look around & reach for toys?