26-year old with weight issue

Question
I recently rescued a 26-year old horse whose previous owner had fallen on tough financial times. Before I got him, the horse apparently had a tooth issue and had difficulty eating and became quite thin (the thinnest living horse I've ever seen!) The dental issue was fixed and he was able to eat but he was not gaining.  I took him and his 2 healthy stable mates and soon discovered that they had been pushing him away from his feed. I also discovered that she had been giving him only 10% sweet feed.  I feed him in a stall so he doesn't get run off and I have gradually switched him to Bryant Feeds Senior (14.5 protein, 12% fiber, 5% fat) and add weight builder (30%)plus free choice hay and pasture.  He has done well and his ribs are starting to fill in.  I have recently discovered that Patriot Feeds makes a Senior feed with 7% fat, so I'm considering switching, but don't want to risk messing with a good thing!  I'm also not sure how much I should be feeding him - I am guessing he would be an 800-900 lb., horse if healthy, but I don't want to overfeed him too soon.  What do you recommend?

My second concern with him is that his hair has rubbed off on his spine and withers.  I'm hoping that will come back as his nutrition improves, but I'm concerned about cool months.  We live in Weatherford, Texas so our winters are typically mild, but last year we had record snowfall.

I spoke with my vet about him in passing while treating another horse, though he has not seen him - he stated that if I was able to get the horse to gain weight in this 110 degree heat he felt there is a good chance he will bounce back.

Thanks for your time and attention to this matter!  

Answer
I think the feeding program that you are using is just fine. You are making up for the lower fat by adding the supplement. Keep up the good work!

His hair will grow back when he gets healthier. It will just take some time.

You are really doing a good thing. Old horses are great. It is sad that they give their whole lives in service and then get discarded when they are old. I am happy to know there are people like you that will give them a decent end to their life. Not all horses get so lucky.