Is Jock on any medication? If he is, the meds could be making him not feel himself. Also, when a cat does not feel well, they tend to withdraw from social life (so to speak).

For instance, my Tony became quite sick at the end of 1998, I knew something was wrong immediately because he was the "social butterfly" of the family. Everywhere I went he followed, but when he did not feel well, he would go down to the basement and stay there.

Thats my thoughts on the matter. Hope Jock feels better soon.