Adjust the temperature in his room. Use thinner cloth, like muslin. Babies do sweat while they sleep, and it’s a good thing, it rids the body of toxins. Just like adults, some sweat more than others. I go to bed cold with all the covers on and end up hot with all the covers off in the middle of the night and then back to cold with all the covers on in the morning, ha! Soaking through a onesie does seem a bit much though! I think generally babies grow out of using the swaddle on their own. Mine started being able to get out of all of them rather quickly so we stopped sooner than others. We just stopped, cold turkey as you might say. He was never a good sleeper, and I could share on that but it’s long and that’s another topic.