It is most likely to be hyperthyroidism - basically an overactive thyroid gland. This will likely make your cat eat more and be more active, but as you have discovered they can get very thin very quickly!
The following link should tell you a bit more:
http://www.sniksnak.com/cathealth/hyperthyroid ism.html
Hyperthyroidism may eventually overwhelm your cat and his heart, so it would be best to take him to the vet for treatment. Your vet will also be able to rule out other causes of weight loss in older cats, including tumours, kidney failure and diabetes.
Hope this helps, let us know how you get on.