Mine is the world's skinniest pig....and every human female's dream - world's least fussy eater, eat 10x more than any warmblood, barely exercise, and lose weight. However he has done well on fibre cubes, alfa a oil, linseed (makes a world of difference!!), and a 50 50 mix of haylage and hay. In terms of supplements I have him on protexin gut, MSM, FP hoof and skin, natural vit e (makes a world of difference), and dried oily herbs (not sure if related but he seems to have a rounder bum and looks better over the hips since being on the herbs. Hard to explain what I mean here!). Unfortunately he has very very poor pasture grass so have done the equibiome test to see how I can help him out more. I have decided if needed, I would add oats but he seems to be maintaining. I have in the past, taken him off the vit e and usually a few months later he loses weight....put him back on and within 2-3 weeks looks great again.
Back home he did fantastically well on hi fat hi fibre, hay cubes, beet pulp and a ton of hay (we don't feed haylage back home unless you want to play with fire with botulism). He has done well on beet pulp here however the kind of beet pulp my yard (now) uses, seems to cause sarcoids so I avoid it now.