Thanks so much for your replies.
I suppose it's the fact that over the past 6 months or so he has sweated a good few nights. Some nights he would get a towel and wipe himself but wouldn't feel the need to change the sheets. But then last night was the second or third time he had to change them.
I know there could be a lot of reasons for it. Some of the ones you mentioned Carys are probably possibilities and some others are definitely not.
But of course I'll always jump to the worst case scenario anyway - that's what I do best