As stated above, senior horses still need care whether they are being used or not. Dental exams are very important for these guys as they tend to have more dental issues than younger horses---fractured teeth, poor wear patterns, loosing teeth, etc. Having their feet trimmed is still also necessary--hooves don't stop growing just because a horse isn't being worked. Vaccinations and deworming are also still necessary.
As for feeding, grains may or may not be necessary currently depending on the age of the horse, what kind of shape it's teeth are in and how well he maintains weight on just forage. But at some point it's highly likely that you will need to supplement his forage with anything from a ration balancer (just to add vitamins/minerals that are not at high enough levels in hay) to a complete senior feed to provide the majority of his nutritional needs due to changes in his teeth and digestive tract from age. You have to sorta "play it by ear" and adjust the diet to fit the situation as you see changes in your horse's condition or changes in the forage that he has to eat--like going from good grass to hay of varying quality.
Again, just because a horse isn't in use doesn't mean that he doesn't still need the basics of care and good nutrition.
Licensed Veterinary Technician