Very common. It is now possible to buy Viagra over the counter, or online. Buy from a reputable company such as Boots, Superdrug or any high street chemist. Keep away from dubious online adverts.
He should speak to his GP first to make sure that Viagra is suitable, the GP may even prescribe it (the NHS allows 4 a month to be prescribed). His GP may want to check his prostate - if it is enlarged it can cause erection problems (doesn't mean he has cancer).
It could be psychological. Is he depressed? Drinking too much alcohol? Over tired? Call a halt to intercourse for now, take the pressure off both of you. A bit of break might do the trick