Re: HELP! What fits a fat baby? *added pics*
Preston's Pants. They have a belly dip for buddha bellies, then come up in the back a bit to prevent plumber's crack.
Colleen will work with you on measurements to an extent - her Skinny Large is made specifically for fat thighed slimmer kiddos; you might do well with her older "medium long" option. The Skinny Large may still work for you though on a larger snap setting then what we're currently using - the thighs are honestly some of the most comfortable I've seen.
We also do well with the size large BG AIO's, but if he has a large belly you may not like how far apart the tabs are/might not fit well.