My Cat has returned after a month but has changed :(?

About a month ago our landlord came up to me and my wife and told us we were no longer allowed to have 2 cats in our apartment. We came to a very sad decision for both of us and decided to find one of our cats a good home. After we found him a home, we though was great for him, we received a call to inform us that she was having behavioral issues with our cat. She said she was going to take him to an animal shelter. We were saddened things didnt work out for her but we didnt want him to be sent to an animal shelter. We picked him up soon as we heard and returned him home. He was only gone a little over a month and when returned he started acting so weird. He would hide from us, and hiss at his brother. It was so shocking to us because he had NEVER hissed since we had him. We began petting his belly as it was his most favorite, and he hissed at me so loudly! It also seems, when i walk over to him he cowers in fear!!! My guess is someone was hitting him which really upsets me so much! It was a big mistake for us to have let him go but we could never give him up to a random animal shelter and only had his best intentions. My question is, will he ever go back to how he used to be? We want our old cat back and will never leave our sight again.