I have a 15 year old cat, who has always been quite thin but in more recent months he has lost even more weight. Now he would be well under 1 kg. He is always hungry. I feed him at least 3 times a day and he loves milk. He is regularly wormed. Do you know what might be wrong with him?

If I am correct your cat weighs 2 pounds. I would see the vet asap. Just to make sure he is not in any pain, 15 is quite old for a cat. If he were my cat I would leave food out all of the time so he could eat whenever he wanted. Maybe the vet will have some answers for you.