I would start your horse on Cool Calories 100. Its 99.5% crude fat but it wont had fat where you don't need it. Just add a couple scoops to your horses grain once a day. I would also reccomend collection excersises. Lots of them. And lots of up and down hills out on the trail. All that seemed to help a tb I was leasing.
THis is me on our morgan mare .. just relaxing down the road notice she is not reaching up under herself with her hindend and her back looks you instead of rounded UP ... her nose is out and way in front of vertical
How about some long and low work? At the walk, trott, and canter, ask the horse to relax and reach. Do collection work. Do trotting poles. Layout 4 ground poles on a large circle. Do lots of transition work. Also, something I find very helpful are transitions on a 20m circle. Ex. Do about 6-10 strides at the canter and ask for a trott, do 6-10 strides at the trott, and then canter, then halt, then walk, then canter, etc. Make sure you do this in both directions. Increase your time slowly and don't ask for too much too soon.