Most of the time, it's just lack of basic hygiene. Washing his hair, washing his clothes. Not bathing very often. As we age, our sense of smell isn't as good, so he probably has no idea. Maybe you could ask him if you can do a load of laundry for him, or if he needs anything from the grocery like toothpaste, deodorant, or soap. Or just pick up some supplies for him. You might even gently ask if he feels safe showering. A lot of older people are afraid of falling in the shower, so they just stop bathing. Maybe a bar in the shower would help him feel more secure.