My LO has always slept on her front. I'll recommend going for a bigger size nappy, it worked for us x
Is he in overnight diapers? They tend to be a bit wider and more absorptive. That's also how we've been deciding to size up for our kiddo, is when he has 2 leaks in a row we size up. He's currently at a size 5, at 11 months old which he's within the range of, though by weight technically he could be in as low as a size 3 but a size 3 he'd leak constantly and we could barely do them up cuz he's got mamas thighs lol. But size 5s he doesn't have any leaks and they fit him comfortably
Put his bedtime nappy on backwards x