My bub can roll onto his side but not onto his belly yet. He has started rolling onto his side during nap time. Should I stop swaddling him?

How old is your baby?
Is he in a bassinet or cot?
Red Nose general information in relation to rolling says:
It’s essential to stop wrapping as soon as baby starts showing signs that they can begin to roll, usually between 4-6 months.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

The following is recommended when baby is starting to roll:

Always place baby on their back to sleep.
Transfer to cot if not already.
Ensure cot mattress is firm, well-fitting & flat.
Ensure there is nothing loose or soft in the cot.
Use a sleeping bag now that has appropriate size neck & arm holes if appropriate….. cease wrapping if you have not already done so.

Red Nose has the following information on web site.

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 lamb’s wool, 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.com.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position