Just a suggestion about his feed, some horses gain more weight on a higher fat feed, like Nutrena XTN, which is 12%protein and 12%fat. I fed that with some corn oil and had really good success in my growing TB. Plus fat calories don't make them hot which is a nice effect too. If you live in an area with hot summers corn oil can go rancid, and we've found Cool Calories to be a good substitute. It's basically de-hydrated corn oil, it has no taste, and almost no texture when added to feed, so I've never had a horse refuse it. Plus it's a smaller amount, most horses get like one or two supplement(like Acti-flex or hoof secret) sized scoops of it, and it's worked wonders for keeping weight on the 24yo TB and the 17h Appendix mare(hell hath no fury like a red TB mare)

Just my two cents, I had really good luck with it so I thought I would pass it along. I think a few people said XTN made their horses hot, but I think they also were on a lower protein diet before changing too.