My 5.5 month old rolls and rolls around in his cot. He has started putting his arms through the holes and uses his feet (throw a sleep suit) to push off. I am worried he will get his arms and legs caught but at the same time very worried about putting up the even thin bumpers, etc because of SIDS. Sometimes I find him right up along the edge of the cot asleep. Any suggestions?

Most babies will be rolling & moving around the cot by 5-6 months of age.
This is normal & does not need to be stopped, however the following is important:
Ensure that the mattress is firm & flat & there is nothing soft or loose in his cot.
It is important not to use bumpers (or other soft padding) in the cot, as he is learning to roll.

Many babies will roll into the sides of the cot &/or get arms or legs through the slats, particularly when they are learning to roll.
Most babies will just go back to the tummy or side, their position of comfort, if you try to roll them back.
Always continue to place baby on back to sleep.
Increasing tummy time when awake may also helpful.

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

Parents are generally concerned about the potential for injury from the infant bumping up against the cot sides, or getting an arm or leg stuck between the slats.
United States Consumer Product Safety Commission data has shown that infants suffer essentially no long-term injuries from contact with cot slats.