We have a chi with hypothyroidism that we have essentially starved to death for the last 2.5 years since we adopted him. He was extremely overweight when we brought him home to begin with, but undiagnosed on his health problems. He has continued to gain weight even with every precaution to limit his food intake, exercising him.
Three other dogs we have are perfectly normal weights. Sometimes it happens.
People with hypothyroidism get thyroid supplements, maybe you can get some for your cat - proportionate dose for his size and weight, of course.
Has he been checked for Cushing’s Disease? Even Thyroxin has no effect if that’s what it is.
While not rare such situations do happen. You are obviously a good owner