I can only say that my husband has this problem too and he uses Extagen which helps him. He says that it doesn't have side effects.
That seems the most likey cause. He should have bloodwork for his sex hormones, adrenals, and full thyroid panel. Hormones allow blood to flow! Does he seem depressed/anxious and show other symptoms like fatigue and lack of enthusiasm? If so I think the first thing to do would be to pick up an adrenals saliva test kit to check his cortisol over 24 hours, this is very accurate because of how this changes through the day.