But at 28 you could try him on some Tribulus, its a herb that will incress his libido, his testostrone and his energy levals, but will take time to kick in, and you could also get some Cayenne
pills and some Garlic
pills these will help his blood and open up his veins to allow more blood to his p***s, all helps for a happy sex life.
You can google any of these for info just add ED on the end.