any sport type horses like that, TB, standard breds, are all known to be hard keepers and hard to keep the weight on, just think of the difference between a skinny man or a lean man, the lean man is skinny looking but all muscle, the skinny man is just skinny with not much muscle. The point is, I find my Rhiszo hard to keep weight on too and he is on high fat high fibre. I know what a starving horse looks like I rescued 2 in my life so far! Its hard to get them back on the same track. Anyway, I'm glad that you have been doing all that you can do, sometimes bran (got to give in small doses if not they'll get the sh!ts) and flax with regular feed, that's what helped get the two horses that I rescued back on track. But those horses are hard keepers sometimes, just because they are built for speed.
you can start with nothing, and out of nothing or no way, a way will be provided.