Hi,
Weight loss in older cats is not uncommon, but if he's lost a lot of weight over a short period of time, there is definitely something wrong. Cats this age are more prone to develop conditions like diabetes, hyperthyroidism, liver and kidney problems. If the vet's bloodwork showed everything is normal, s/he has to look further. The only other common cause for weight loss in older cats, is cancer somewhere in the body. I'm not mentioning this to alarm you, just to inform. Intestinal or blood-borne parasites are another possibility, and your cat should be evaluated by a specialist, if your regular vet couldn't find any cause for this weight loss.
http://www.sniksnak.com/cathealth/hyperthyroidism.html
http://www.sniksnak.com/cathealth/diabetes.html
http://www.sniksnak.com/cathealth/renal.html
Right now, you're doing everything right; if your cat is eating, drinking, eliminating and acting fine, that's good, but the cause of this weight loss needs to be found by another vet or specialist who can do more advanced procedures (non-invasive).
I hope all will be well!
Cher
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