i suspect something like noro which can come on very suddenly.
however it also wouldn't surprise me if he is struggling mentally, Kate has clearly has /had something quite serious , he lost his own mother as a child , whilst the circumstances are completely different it would not be surprising if it triggered feelings of fear about his children losing their mother and what that would look like, together with bringing back the trauma of his experience. They are also very much a double act, Kate has been widely credited with helping him to come to terms with becoming King , he may be facing the prospect of her health not being robust enough for her to fulfil this role . His father is ill, we don't know how ill, he has a useless uncle and a brother who has accused his wife of being a racist and is a complete loose cannon. i also feel a bit sad for him, it doesn't matter how much money you have they are still human beings with feelings and emotions .
I have said before that i think William will be the last king and that he will be happy with this .