On average, the untreated HIV infected patient will progress to AIDS in about 5-10 years. Since 1996, the introduction of powerful anti-retroviral therapies has dramatically changed the natural progression of HIV infection to the development of AIDS.
You CAN feed your horse just hay, but you have to up the portions and it also depends upon the type of hay you are feeding. Alfalfa mix is normally the best but there again, you're goinng to have to feed more hay to make up for the not feeding grain when you are riding.
Eliminate all grains, supplements and other feeds you suspect has sugar in them. There are now feeds and hay replacements that test under 11% non structural carbohydrates and are safe for the insulin resistant horse. Finally, get that horse moving and exercising.