First night with a blanket

I feel really dumb for asking this...

How should I put my toddlers blanket on his to sleep overnight?

He’s been in a sleep sack up until now (he’s 20 months) but tonight he fell asleep on my bed unexpectedly after his bath and his sleeping bag was downstairs so I couldn’t go get it. I’ve put him into his bed now but to get him in the sleeping bag I’d have to take him back out of bed as he’s not in a position that I could easily get his sleeping bag on.

I’m happy to use a blanket and was going to start soon anyways, but every time I have he’s wriggled so much that he’s ended up on top of it or with it wrapped around his leg or something.

He is a massive wriggler so I’m a bit worried he’ll end up with it on his head 😂