Is it ok to use a babyhood sleep positioner for a 6 week old baby that is rolling onto his side . This is made to fit under the arms and had a roll either side of the baby’s body to stop rolling .

Red Nose does not recommend the use of any device that keeps baby fixed in 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

There is no need to stop your baby from wriggling & changing his position, but it is important to make his environment safe in light of what he is doing.
Many babies will wriggle & change position & many get to their side at a young age.
Remove his blankets to reduce the risk of his head & face being covered.

Often this can mean that baby is soon going to be rolling further.
Are you still wrapping/swaddling him?
Some babies just do not respond well to wrapping & like their arms free.
Often these babies like their arms free or like to touch their face or suck fingers & will wriggle to get free.
How does he go with tummy time when awake?
Is he still in a bassinet?

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 tummy.
*It is important that you move him from bassinet to his cot & cease wrapping once he is rolling or attempting to roll.
*Baby needs arms free & the space of the cot to roll safely.

Once he is rolling it is important to make the environment as safe as possible for this to occur.
Continue to place baby on the back to sleep.
Ensure cot mattress is firm, well-fitting & flat.
Ensure there is nothing loose or soft in the cot.
Preferable to use a sleeping bag when rolling that has appropriate size neck & arm holes.

https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over