Has he got a skinny back? I'm not sure if it's a boy thing as much as the nappy may be gaping a bit IYKWIM. I have had leaking out the back with all of my newborns and the only way around it if using sposies is to use a cover to help it stay closer to the skin and sleep bub on a towel jic.
If you're doing cloth you can get creative with the folding or use an extra booster.