First thought is he's met someone. A private crush, a mutual emotional affair or an actual physical entanglement.
Start digging because if it is so, he will lie, conceal and deny. And this won't go away.
Second - midlife crisis, middle aged blues, mourning lost youth, what might have been, 'if only, is it too late, is that it?'
Has it all fallen into a predictable dull pattern, with the terrifying prospect of ageing and decline on the horizon? I get that, so JFDI, saying yes to scary things and new things is something we consciously do.
Third - medical. Hormone drops, depression, an undiagnosed condition.
Taking responsibility for his own health and wellbeing is something so many men fail at. I once came home to find my IH googling 'what should I do about this pain in my chest?'
Whatever it is, he's been thinking it all over for a long time, bottling it up, hiding it until now - and you're blindsided. They do this. Time to find out which of the three lie beneath. Once you know, then you can decide what to do.