Hi,
I have a 14 yrs old cat with hyperthyroidism. I found the best advice on a facebook group. You migt want to try there for help and advice. If the food your vet has suggested is a Hills Prescription diet, then in my personal opinion, you would be better off finding a high meat content food for your cat. If you look at the popular brands in the supermarket, they tend to average at 4% meat content!!!! Terrible. I've found Iams, Hi-Life and Natures Menu have much higher meat content, from about 28% to 60 or 70% I think. It is more expensive than the 2 main brands but much cheaper than prescribed food from the vets.
As I have discovered with my own health and doctors and nurses, I have come to research and question what the professionals think is best for me. I then learnt to question what my vet thought was best for my cat. I now have a new vet who is much more understanding, more interested in my cats health and less about how much they can charge me for each visit.
I would also ask what medication or follow up tests they are offering. If your cats bloods are not getting better that could also be a reason for the continued weight loss. Can you test the cats blood yourself? Not sure if cats would let you. I know I've seen a vet nurse giving a test monitor like i use to someone for their dog. Think my cat would never let me near him again if I did that.
Also, the advice given to new diabetics is to cut out as many carbs as possible. Look at the contents of your cats food, increase meat content, even in any biscuits you give him. Ive found applaws cat biscuits have a high meat content.