Beyond his other medical problems and medications, it is pretty common for middle-aged people to have less energy. That is why we have vitamin supplements, particularly Vitamin D and Vitamin B -12. He may need the whole B complex of vities. (Especially if he does not eat vitamin-enriched flour every day, like in breakfast cereals.) He may even be low on Vitamin A, which tends to be drained away by health problems (particularly colds and flus and other common illnesses).
Before you panic over zebras, try horses. Vitamin lack is very common, even with people who already take a multivitamin. (Because our bodies do not process it as efficiently as we get into middle age.)
Eating fermented foods sounds stupid, but some guts really do better with some yogurt or sauerkraut or beer every few days, or even every day. It helped my middle-aged metabolism a lot.
You can also look up foods that promote energy, like some veggies, and figure out whether he is eating and drinking healthy foods all day.