What a great thing you're doing!
I would allow him to roam a single room, a room where things like the bed, dressers and desks have no place for him to squeeze under or behind (as you said, if he's hiding he's not being socialized). If no such room exists, block off his access to getting under and behind these places.
You can keep his kennel in the room as his safe place to run to, cover it partially with a towel or blanket if it isn't a dark area, so he feels safe while in there but it's still an area where you can reach in at any time if needed. Keep working on socializing him in the one room, spend as much time in there (with a tv, books, phone, computer/laptop) so he gets used to everyday things, your voice, and your movements.
That he's allowing you to pet him after a week in a good sign. Keep your movements slow, don't look in his eyes (that's seen as a threat). Try petting him as he's eating, when his guard is down, and don't be too put off by hissing, he will grow out of it as he learns to trust you but it'll take some time. He may scratch at doors and windows to get out because the area isn't what he's used to, but for now I would just endure it, the wanting outside should pass. When he seems confident in room you keep him in leave the door open and let him explore for a little while on his terms. Put him back in the room if it seems like he's hiding somewhere. Keep at it until he seems content in most areas of the house, and make sure he has access to this one room at all times.
When I had a feral kitten (16 years ago) he stayed curled up pressed into couch for a long time and he hissed a fair bit (there wasn't anywhere for him to hide). Outside of the first year of his life I can say I've only heard him hiss about five times, when he's extremely stressed. It probably took a year or more for him to feel totally at ease in the whole house, he didn't like the hallway where he could hear people coming and going in the apartment. Eventually he started sleeping in the closet right by the door though, so he got over it.