If it is thyroid, he may need some fluids just now as well. Can you tell if he is dehydrated at all? I thin kyou know how to check this right? Pinch the skin on his back and see if it goes right back in place or just hangs there in the pinched up shape.

I found a good canned food at PetCo, Soulistic Sunrise Harvest, which has a LOT of fluid in it. I am using that for my Sparkle just now, who is early stage CRF, and so far so good she is not needing the sub q fluids.

You may want to add some canned to up his fluid intake until the vet visit. Just a thought.