What is wrong with this Shiba Inu?

Over the past few months this dog has developed a lot of skin infections. The vet said they were staph infections similar to the kind Saint Bernards can get. He's been on antibiotics with no positive effect. He has also been trembling a lot. I swear I saw him have a mild seizure once where his forelimbs were out in front of him and his eyes were all screwy looking. He was also drooling which he does not ever do. He seems lethargic most of the time which is very out of character. He has within the last 2-3 weeks started to limp a lot and has a bump near his hip. He's only 7 years old. He ate a lot of cat food when he was younger until I showed up about 2-3 years ago and built an apparatus that keeps him away from that dish. Could that cat food have damaged his organs? Does anybody know from experience what on earth is going on here? Why is he degenerating so rapidly?