Depending on the quality of the horses teeth, I'd go for topspec balancer and conditioning cubes (maybe with extra oil) or balanced horse feeds show mix.
In the past have fed Allen & Page Calm & Condition, Allen & Page Weight Gain, Baileys Topline Cubes, barley rings and barley and all were very good.
there is a brilliant feed out there called boomerang copra (www.boomerangcopra.com). i have seen it used to brilliant effect on aged horses horses with teeth problems, serious cases of weight loss. i have also used it as a maintenance food and a competition food. it is well worth a look at the website (He will probably give you a free trial)
its made from coconut, you soak it for 1 hr cold water, 20mins hot water. it lasts ages, smells like choclate and literally works miracles. it has a high protien level but don't be put off because it contains a apecial lysine that gets rid of excess protien in the body.
Definatley reccomend.