I was having the same problem with my 13 year old TB. He was in a dirt run out and extremely skinny and his coat was totally bleached out. I had him moved to a grassier turn out and I put him on Dumor weight builder and a cup of alfalfa pellets with every feeding. After a month, he’s gained about 150 pounds, his coat is dark again and he’s generally happier. The alfalfa pellets are a little cheaper than the Senior feed (which I was considering) but they’re working just as well. You could also use cocosoya oil to help with weight gain, but I found that weight builder works just as well. Hope this helps!