your old man is funnily enough ‘old’ so he might find the increase in volume too much to take just yet, maybe he should gradualy increase the volume and try not to do to much. Also at his age he can usually get testosterone replacement from his local gp ( i believe it usually comes in patches) and that can help some what in the amount of work he would be able to do.
Watch his diet and make sure he is not deficient in anything especially total calories but im sure you know this. If he drinks a lot then maybe he needs to lay off the alcohol a bit. And get plenty of rest. Not much else you can do but my first point is probably the most important.