i notice you use itti bitti heaps already, so you've already seen how a low rise, narrow crotched nappy sits on him. with one row of front snaps.

have you tried any other DIFFERENT CLOSRUES on him?
e.g two rows of tummy snaps like Fluffy bubs?
(their size large i think starts at 8kg)

e.g side snaps like swaddlebees pockets?
(their size large is 10 - 17kg)

many OSFA nappies are pretty darn wide in the crotch, but ittis are narrow crotch (and sized), so sounds like that's what he suits.

haven't seen your bub, but the buddha belly seems to be pretty common with little ones e.g nappy sitting much higher at the back than it does at the front.

i might have something for you to trial on him, just PM me for details. Do you have any pocket stuffers, trifolds etc?