I would make sure that they are both on a worming schedule. With the thoroughbred most of the time they are hard keepers, especially when they are being worked. Making sure they have hay all the time is great, but you want to be careful with the amount of grain you pump into them bc it could make him really hot and a handful. You might want to try a weight supplement or you may want to try beet pulp. You can also try putting him on a sweet feed, like half sweet feed and half pellets that way you are adding fat and he might not get extra energy.
Its good that you havent seen any more weightloss that means it may be working, and it will take a while longer till you start to c them putting weight on again.