My 5 1/2 month old takes a dummy to go to sleep. He is now rolling onto his tummy or side to sleep. I roll him onto his back whenever I see him do this, but not all the time, as I can’t stay up all night. Is it safe for babies to use a dummy when sleeping IF THEY SLEEP ON THEIR TUMMY OR SIDE?

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 

Ensure mattress is firm & flat & nothing soft or loose in his cot.
Continue to always place your baby on back for sleep.

Continue to gently roll him back when you see him on his side (or tummy)
Parents are not expected to sit up all night watching their baby.
Once he is able to roll both ways, still place him on his back, but let him find his position of comfort. (see linked article)

Is he able to roll & change his position during 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)
This might mean increasing frequency of tummy time, rather than how long he is on his tummy each time.
Is he rolling from tummy/side to back during tummy time, holding his head well &/or reaching for toys?

It is ok for baby to have their dummy when sleeping, including when they are rolling.
In fact, in some countries (not Australia) it is recommended that baby sleep with a dummy to assist in reducing the risk of Sudden Unexpected Death in Infancy. (SUDI & SIDS)