With semi-feral (or just scared/overwhelmed) cats, I have found using a large dog crate or one of those tiered cat cages is very helpful for giving them a safe space while they adjust to the environment of the household. When they can hide and avoid contact they often remain freaked out so the crate creates security while allowing the cat to observe and acclimate.
I have a giant sized wire dog crate that is large enough for a litter box, a small enclosed sleeping/hiding spot (upside down box with an entry hole works fine), food and water bowls, and a soft sleeping/resting area outside of the box. I often cover the back half of the crate with a blanket for enhancement of the “safe space” feeling for the cat.
This kind of set up gives them sufficient opportunity to have a safe place to withdraw while they remain able to observe at least part of the household environment and have easy access to food, water, and litter. Depending on the situation, I have started with the crate in a quiet room away from the other animals. In other cases (where the cat was more anxious than terrified and my other cats and the dogs were very good about leaving the newbie relatively undisturbed), I have set up the crate is a part of the house with a bit more activity. Offering special treats to entice the cat and allowing the cat to initiate any contact helps to reinforce a positive, non-threatening relationship with the caretaker(s).
Eventually the cats figure out the routine, become curious about what is going on and begin to engage with the caretaker. Since this is not a feral cat, but one that was traumatized by the transitions between original home, shelter, and new home, he should come around fairly easily.
Good luck with this kitty.