Rant! Diaper help!! Its barely 10am and my baby has already gone through 2 onesies because of poop…

Rant! Diaper help!!

Its barely 10am and my baby has already gone through 2 onesies because of poop coming out from the top of his diaper. He is 4 months old, 16lbs and wearing size 3 Huggies Snug&Dry. I don't get what I'm doing wrong. They seem to fit him well around his waist and I always make sure the back part of the diaper isn't tucked in. I'm a ftm so I really don't know much. Any advice would be welcome!