In all honesty I would get hold of a fleece nappy cover (Maz-a-licious is good ) and put that over the top. The fleece will hold in any leakages and will still breathe so he doesn't get any hotter and sweatier. I have a M size that should fit if you want me to send it to you to try? sounds crazy I know. I do this overnight when we go away and I also recommended a friend do this with her NB when he was leaking. It works