He won’t sleep in his crib anymore

Delete this if not allowed. So by the title, he’s obviously been cosleeping with me. It’s been cold at night and he’s not old enough for a blanket. He’s 8 months and I’ve been recommended sleeper sacks. However, he rolls in his sleep. Also my fiancé won’t even let him have a fitted sheet on the mattress making it really cold. So how can I keep him warm and not super attached to me because I don’t want him on me during the summer.

Edit: the baby’s pediatrician said not to have a sheet and just the mattress and I’m like… why? 👀 didn’t even give me an answer!

Edit PT 2: Thanks for all the suggestions of a sleep sack, and my partner agreed to a sheet with a zipper. He slept 9 hours in his crib! It was wonderful. 🥹