I tried Fat Cat on my tb and it didnt really work it made him very shiny put he didn't put on any weight. What did work for me though is Focus WT and rice bran. My friend swears by Calf Manna I don't know how good it is for horses since its meant for calves. But yeah anyways have his teeth checked, deworm him and if you have enough money do a round of ulcerguard being a tb it would make sense for him to have ulcers.
My OTTB had the same problem with weight. But I know in the beginning that he was dealing with absesses in his hooves so that definitely contributed to the weight problem.
Once those were cleared up we put him on TriMax High Fat. We gave him half Trimax and half Oats in the morning and at night. He was turned out to graze for the rest of the day. We also made sure he had a constant supply of Hay (alflafa).