Firstly, I have to repeat what others have said ... you are an ANGEL to take this poor cat on.
Now I don't know how feline medicine relates to human medicine, but I'm just posting this for what it's worth.
"... he has so many of the symptoms that it doesnt make sense: He eats ravenously, drinks constantly, has severe weight and muscle loss, frequent urination, racing heart rate, etc...can a cat have a normal T-4 and still be hyperthyroid? "
This describes to an absolute T my closest high-school friend when she was an undiagnosed diabetic. These are ALL of the symptoms she had, every last one of them. So my question is ... has his urine been checked for ketones / have his blood sugar levels been checked?
Found a website at http://www.felinediabetes.com/
so you might want to check this out.
Hope this helps,
Catherine and the fearsome five