Is this cat of mine feeling miserable..or just getting old??

I have a 13 1/2 yr old cat who lately seems to be very very low key, moves rather slowly, especially when getting into position to lay down or just walking in general. Yet he makes a pretty decent trek over to the food bowls and expresses great interest when its time to eat, along with all the other guys. He just seems so apathetic and lazy, and he's not a fat cat either. He has always been very much a light weight, in the 8 lb range during all his annuals, but I swear he is losing weight, probably down to 6 lbs or so. He seems to have no fat under his skin at all and very bony. He was 7 lbs a couple of months ago at the vet. In the last couple of years he had surgery to remove a small carcinoma in his back hip, he had an ear hematoma, and last summer he was bitten by a racoon(he snuck outside), and I got him immediate assistance with a booster rabies shot and some antibiotics from the vet. He doesn't whine or yelp if I touch him anywhere to see if he has pain or anything. I am just trying to save myself another vet visit if I can help it. I have always heard that when they dont eat or drink that's when to worry. Unless that is an old wives tale.
Anyone else have or have had elderly cats in similar situations, and what have you done about it, if anything? Thanks!!

 

With a history of carcinoma and his given symptoms (slow moving, lethargic and losing weight), I would definitely take him to the vet.

Yes, 13 is a good old age for a cat but like I said, with his medical history...it could have come back. If this is the case, he may live another 5+ years with treatment.

But I'm no expert by any means. This is just my opinion.