My son is 15 weeks, started rolling belly to back every now and then. In the bassinet overnight he looks like he might roll back to belly, so we’ve brought his arms out of the ergo pouch. I’m scared because he can’t roll belly to back very well. What can I do to prevent him rolling and not being able to roll back?

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

Now is the time that you stop wrapping & move him from the bassinet to his cot.
Baby needs arms free & the space of the cot to learn to roll safely.

How does he go with tummy time when awake?
**Always continue to place baby on back for sleep.

Red Nose has the following general information now that baby 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