My 6 week old has already started rolling over. I have started putting him in a transition sleep suit where arms can come out etc (love to dream suit) but his free arm keeps startling him awake. Any suggestions on keeping him safe sleeping in his bassinet but also asleep through the night and during his day naps

Most babies will be rolling over by 4- 5 months - some earlier.
This is very normal as babies grow & become stronger.
Many babies eventually prefer to sleep on their tummy.
This is not something that needs to be stopped. It is important to make his new environment as safe as possible.

Now that your baby is rolling it is important that he is no longer wrapped or swaddled.
This includes using any swaddle suit that has arms in.
It is also important that he is in his cot for sleeps now, rather than his bassinet.
He needs the extra space & arms free to roll safely.

Also, ensure that the cot mattress is firm & flat & there is nothing soft or loose in his cot,
Always continue to place baby on back to sleep.
Increasing tummy time when awake may also helpful.

Red Nose has the following information now that your baby is starting to roll.

If you are using a bassinette, it is time to transfer baby into a cot as soon as they first show signs of being able to roll.
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