Sad Kitty

I'm a bit off at the moment, because my beloved cat isn't doing so great. I took him to the vet a few weeks ago because he was losing a lot of weight, and it turned out he has an overactive thyroid and he's been on meds ever since. HOWEVER he had his follow up last Friday and he's lost more weight, and he's not eating much now. I'm worried. I know he's really old but I'm just not ready (though, well I ever be?) to let go yet. He's honestly the sweetest cat I've ever known. He loves to cuddle and he purrs all the time. I do feel that it could be ok though, because the vet thinks he's still happy, so hopefully even if something is wrong it can be sorted out!