Could have sworn I had a fool proo system.
Wait 5 minutes after a fart before changing him. Gives him time to work the rest out so we don't waste diapers or effort. It's brilliant really. So after a large wet one, we listen to him grunting and squeezing so loud you can hear him in the next room, for at least 5 minutes. Gotta listen closely too in case you miss a fart. If there is a fart, reset the clock.
Well, that doesn't always work. Changed his diaper, and luckily I got the other one on before a massive explosion. Not a bad success rate though. Perhaps make it 10 minutes next time?