Welcome! My name isXXXXX am a 2003 graduate, and currently a Medical Director of a veterinary hospital.
I am sorry to hear about this concern for Mister.
Yes, a few thoughts.
1. You need to inquire with the vet if they performed a thyroid level on that last blood test. Even if it were normal, make sure to check it again.
A extremely common cause for weight loss in middle to older age cats is hyperthyroidism. They can even be eating normally and still lose weight.
Unfortunately, another concern would be a cancer cause. Yes, even with a perfectly normal blood test when last checked. So, I wouldn't be surprised if at some point the vet recommends either a abdominal x-ray, or even more likely, a ultrasound of the belly to look at the organs, as well as, the intestinal and stomach walls and layering. Even with no vomiting or diarrhea, this is a concern. What is Abdominal Ultrasound in Cats
To try and help improve the appetite ask the vet for a stimulant medication like cyproheptadine, given up to twice daily as needed, or better, Mirtazapine, given only once every 3 days.
Considering the huge appetite yet losing weight initially, the thyroid issue is suspicious.
I hope that information has been helpful.
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