It could be a multitude of things. Diet is always something to scrutinize, as fatigue can be caused from lack of proper nutrition, but it can also be a more indirect result of candidiasis, leaky gut and other disorders which result from such.
Does he have any other symptoms? Skin rashes, irritability, insomnia, listlessness, tummy troubles, headaches, brain fog, restless legs, food allergies, seasonal allergies?
If he has absolutely no other symptoms then a multivitamin might be enough to perk him up, but if he's got one or two or more of the above symptoms (or maybe some others I've not listed), you may want to dig deeper.