Does he (and grandma) take a good, whole-foods based daily multivitamin/mineral supplement? It sounds like he could be low on specific nutrients. Many elderly people are, first because they eat less as they age/food just isn't interesting or tastes good to them, and second because as we age, a lot of the vitamins, minerals, amino acids, and enzymes that our bodies took for granted as we grew up and lived our lives are now either depleted or getting depleted, just because of aging.
Rainbow Light 50 plus Mini-Tab™ Age-Defense Formula -- 180 Tablets ($24), or MegaFood DailyFoods® Men Over 40™ -- 180 Tablets ($65--very good company) would be the place where I'd start--both formulas have just about everything he would need to get everything back in line. You can find both at vitacost.com, a very good online store for both supplements, personal care products, and allergen-free foods. I have no affiliation with them other than to let others know of the wonderful products they carry, many of which are very good supplement companies. I have used them for over 5 years to buy all of my supplements and have found that I have saved hundreds of dollars a year.
Is he on medication for his heart? At his age and if he's on any pharma drugs at all, he should also be taking CoQ10, a co-enzyme that is very important to the body, as it provides energy to the mitochondria, the powerhouse that runs the body's energy.
And, as another poster mentioned, if your grandfather isn't drinking enough liquids/water, he could be dehydrated. Dehydration will make a person dizzy and disorientated. Of course, the hospital should be able to tell that from any blood work they run--but that doesn't necessarily mean they'll mention anything to you about it. Just make sure that Grandma and Grandpa are drinking at least 10-8 oz glasses of water a day. Drinking and urinating enough will also make sure they don't have to deal with or worry about urinary tract infections or bladder/kidney infections because the body is being cleaned out.
Another tidbit your grandparents might find helpful: drinking 2 tsp of pure lemon juice in hot water every morning upon wakening and before eating is one of the simplest ways to cleanse/detox the liver, gallbladder, and kidneys. If everyone did this, few people would end up with stones in their liver, gallbladder, or kidneys. Or the stones would pass through and not build up and cause problems.