also, try superworms and try lightly tapping it on the front of his mouth to get a rsponse. If you do feed him mice make sure they are pinkys. the fur on hoppers is hard for digestion. Mice are high in fat and I have read that they do not have a good nutrition ratio. i give mine mice as a treat every two weeks but I feed them worms one day, crickets the next, and a dubia the day after. variety is key and make sure to read up on calcium dusting. sometimes less is better than more.
It seems like his aestivation (hibernation) was pretty long. He is most likely still bouncing back.
This frog does need water. i usually put mine in a warm water dish and let him soak. he likes it a lot and sheds his skin.
I am sorry to hear that this previous owner did not do the research to have a specialty animal. I am glad you are doing everythign you can to give him a good life. just make sure to keep posting on here. I have had great advice coming here with my questions. good luck! message me if you have anymore questions