jerry, for better or worse

We're a bit worried about Jerry the cat. We alternate between being worried about him and wanting to wring his little cat neck for being highly irritating. In between times, he is occasionally very cute, as per this picture. He's pretty old. We're not sure how old because he came to live with us as a runaway or a cast off or something. I won't say he was a stray because he was neutered and knew his way around a soft chair. He's at least 15, anyway. Jerry has always been a very fat cat, but now he's extremely thin. He used to have a hearty appetite, but now nothing suits him except the odd bit of this and that. The other night he had macaroni and cheese for dinner. Last night he ate some pizza. I was giving him salmon for awhile, after he rejected all catfood and did not eat for three or four days. But now he rejects the salmon, too. So it's back to the vet for Jer, to see if there is something really wrong with him. He had thyroid tests last time, because he'd lost so much weight, but his thyroid is ok. It's just hard to know with cats. They're not big on explaning themselves.