As others have said check to see if he needs his teeth floated. Talking to your vet before you start putting him on any additional grain is a good idea. Even if you just call your vet and talk about the problem and potable solutions. If you do ask the vet where to go form here maybe make a list of things you want to talk about. I have my horse on a Seinor Active food and he gained weight very fast, as will as other people have made huge improvements with there easy keepers at my barn. If you have any questions about any of them I would be more than happy to answer.
Can you see a reason for the weight loss? Has he always been a hard keeper? Is his worming up to date? Is there any possibility of pain? Is his teeth up to date? I know senior horses can been more difficult to keep weight on, but a sudden loss in weight might warrant a vet visit.