What should I do about my male friend?

Back in 2007 and 2008, my male friend (with whom I used to work in 1998) became the best of friends. We would chat and vent about everything and anything and would talk daily. Since late last year, my friend now only talks about his cats and how he hates his job (the very few times he chooses to talk) and when I would respond to him, he shuts down for a long time and doesn't talk for many days. He insists that I am doing nothing wrong and that nothing bad is going on in his life to precipitate this, but he insists he likes me and wants to be my friend ... but why all this hot-cold nonsense? Does he just see me as a "friend of convenience"? (My friend will not talk about it, so initiating conversation is not the answer here.)