When you get a new cat. You should put the cat into a small confined room. Everything is new to your cat in the house. If you have anything from his former owner (which you deem clean, no bugs, fleas, etc) that the cat use to sleep on, it might be great to have that.
I generally put the cat in my bedroom. This has lots of my smells in there. Clean clothes, dirty clothes, etc. The one downside is the litter box, and the litter box at night can/cannot be noisy depending on the cat. Also eating crunching food can be noisy at night. Kind of a small sacrifice.
Is any food disappearing at night? If some is disappearing, then eating is occurring at night. If you know which type of food they use to eat, you might want to try that food, or call the owner if possible. Since your cat is 5 he could be really picky and only want a certain type of food. Some cats are really picky; others arent and will eat any type of food.
Do you have the food/water near the cat since he has relocated behind the couch? You could even get a small bowls and put it behind the couch near the cat w/ the food/water.
Eventually you can move the food and litter box to the location you want.
Right now, I'd put the cat in a smaller confined room if possible.
If you don't want to do that. Put the food/water near the cat to encourage eating and drinking. I'd move the litter box near that location too.
At the beginning, keeping the cat in a confined area gives your cat security, and won't be so overwhelmed in his new environment. He can explore the new area, and not be overwhelmed.
If you want the cat to get use to your smell and scent.
Put a well soiled (smelly, sweaty) t-shirt that has your sent on it, with the cat where he is hanging out. This way the cat will get use to your smell without the big person being there. Works great when pet sitting a cat that is afraid. They get use to your scent when you aren't around at night, next day they know who you are.
Depending on how social the cat is and or how aggressive the cat is. You could try picking the cat up after you got him from behind the couch. Then try holding the cat.
Its easier if the cat doesnt have front claws, of course be careful about the teeth, some cats bite. If he doesnt want that at all. Stick a hand back there and let him smell your hand. Dont make eye contact, just stick your hand back there (again if there isnt aggression, be careful if there are clays). May need to start kind of far back, and then inch in slowly. Im talking this could take like 30 mins all depends on the cat. It still may not work, some cats take a long time, and he was removed from his home and is scarred. If the cat comes near your hand, kind of wait until after he makes contact and sniffs it directly.
I would try to pet their back first, unless the cat rubs his face on your hand. Pet the back area first; its less formal, and more likely to be acceptable. As the cat becomes more comfortable, coming near the top of the head, and chin.
Depending on the cat, rubbing his belly, or touching feet could be means of war, and could illicit a nip or swipe of the hand. We have one that loves belly rubs, and one that will nip at you.
The smelly shirt would make this happen faster the next day. If you have lots of family members again that could be overwhelming depending on the family the cat had before.
For bedding. You could put down and old towel, or an old blanket you have delegated to pet status, and dont care if it gets furry.
Has the pet been to the vet recently? If so, do you have the weight of the cat? This could become important, if the cat takes a while to eat. You dont want a cat to loose too much weight.
Good luck, and if you have anymore questions feel free to ask.
I love my kitties; you just have to be patient.