Advice on cooling summer nights

I'm waiting for my sleep sack but while I do I need some tips and tricks that will do.

it's still reasonably hot here. like I still sweat at night. it stays hot up until 2 then the temp drops a very quickly.

I don't want to put him straight up in a winter onesie for fear of overheating. I do when I wake up for his 2 am feed but the thing is his sleep/eating schedule is so all over the place now so like this morning when he woke up it was already pretty chilly (it was after 2) and hes was cold. I know I should probably set an alarm but I keep forgetting.

is there any hacks or tricks I can try up until the sleep sack arrives.