Thanks for all the advise! The good news is he did potty last night. I coaxed some water into him before I went to bed and then left him to explore the living room by himself. When I got up, he did go hide again, but I saw that he used the bathroom. So hopefully this means we're making progress!
I don't have any feliway but I can look into it. We spoke to the previous owners and got his food, they seemed equally concerned. He hasn't really moved from his spot. I contact the vet tomorrow to see what they suggest doing.
I've been doing research to try to figure out what to do and I saw a similar thread on this topic. I recently got a new cat, his name is Gus Gus. He's a five year old orange tabby. The home we got him from was getting rid of him because of their two large dogs. I guess they were keeping him...