My 8 month old has sweat a lot his entire life as well. He is perfectly happy sleeping in onsies, and if I try to cover his legs with a light blanket, he always kicks it off. When he was smaller, he needed to be swaddled, so we just used a thin blanket. I've also heard of people cutting a sheet into squares and using that for swaddling. Don't be afraid to swaddle him in just a blanket (or sheet) and his diaper. It's totally normal! Don't worry about it too much.