I have a 7 month old who moves a lot in his sleep, often waking himself multiple times in the night because he’s hit his head on the edge of the cot. Are there any cot bumpers that are safe, or other methods to protect his head and preserve his sleep?

The only bumper that would prevent a baby from hitting the sides of the cot, would be a thickly padded bumper around all the cot.
Red Nose does not recommend using bumpers for bay because of the risk of suffocation.
https://rednose.org.au/article/at-what-age-can-i-introduce-cot-bumpers-and-pillows
https://rednose.org.au/article/baby-products-you-dont-need

Many babies will roll into the sides of the cot when sleeping.
Does he wake/cry when this happens?