How do I keep him on his back??

Hi there.
My little one is 17 weeks and has discovered the delight and frustration of rolling on his tummy (as have I...). When I put him to bed for sleeps he rolls onto his tummy, comando crawls to the top of his cot, gets VERY frustrated and cries (as do I some days). I have tried wrapping his arms, putting a sheet over his belly and tucking it in to try to pin him down but nothing works. I know you can buy thingys which can help, but I'd rather save the cost if there are any suggestions. I'd greatly appreciate any advice.

Have you tried rolling a towel and putting it next/under him to stop him rolling over? You may need 2 towels if he rolls both ways (my DS1 only rolled to his right, never to his left).

Hmm i WAS going to suggest that it not be a worry, but considering he doesnt roll from tummy to back then i know how frustrating it is...
Although my DD can roll tummy to back, she chooses not to most times, so i have to roll her myself. Luckily she doesnt do this in her cot! And sometimes she doesnt mind being on her tummy.
I would suggest trying rolled up towels, but your DS being mobile, i dont see them working, my DD isnt as mobile as your DS yet, and she still manages to move the things we bought to keep her in position and gets out of it.
Im not sure what else there is you could do, but i do hope some one can come in and help. I know how frustrating it is, my DD has pushed herself up to the end of the cot in her sleep a few times, and bumped her head and woken crying.

I have to keep my son in a certain position due to a flat spot on his head & he's started to roll so it is a challenge. I tried towels etc and they didn't work for me. In the end I got a Sassy sleep positioner from Baby Bunting (around $35). Once I place him on his back he's there for the night. It's only useful apparently til 6 months. Mine also has a pillow thing attached to direct his head but there is one that has just the back part and it's only about $25 in Big W. I also didn't want to spend the money but had to in the end.

The towels don't seem to work as he gets around them but thank you for the tip. Satya, thank you for advice I don't have a Baby Bunting close but there is a Big W down the road so I'll try tomorrow.
Thanks again girls.