How do I make scared cat more comfortable around other animals?

I feel so bad for our little "guest cat" and I don't know what I can do to help him. He's three years old and has been living as an only pet with my husband's niece his whole life. She was renting our place and we just moved back in here with all our animals and the niece have found another apartment.
The new apartment requires a $200 pet deposit and a $25 a month pet fee and the neice doesn't want to pay this so she's planning on smuggling Jack, the cat, in once she's "checked the situation out" to see if it would be possible for her to have him there without the landlord knowing. In the meantime Jack is staying here.

I have my opinions about how she's chosen to do things and that she prioritizes Jack so little but there isn't much I can do about that. I feel really bad for Jack though as he is clearly unhappy and stressed out. We have two dogs and a total of twelve cats (mostly fosters) and Jack does not feel comfortable around them. Most of the time he sits on a shelf in the closet and only comes down to use the litterbox. He has come out to the living room a few times but he gets scared pretty quicly and takes off back to his shelf. He doesn't know how to interact with the other cats so he hisses and swats at them a lot which makes them not be too friendly with him. When he's not hissing and swatting though most of the cats accept him but he's still nervous around them. I've asked the niece to please bring him home with her where he can be comfortable but she doesn't seem too concerned. I have a feeling he will be here for a while.

I could really use some advice on how to make Jack more comfortable around here. Is it best just to let him sit up on that shelf with his food and water bowls or should I take him down to try to get used to the other cats while he's in my lap? Any suggestions would be great. It's not right for him to have to live like this.

Thanks.

Give him his own room all his own and keep the other cats out. Some cats just aren't sociable and forcing them to be can cause them to be stressed out which leads quickly to health issues. You can easily make a screen door for the room using pvc pipe as the frame and pet proof screening. This way he can see and smell the other cats and decide for himself if he ever does want to come out and play or not. but let it be his decision as that is best.

Did your niece give you his posessions? Cats seem much more comfortable if they have familiar sents around them, in regards to moves or location changes.

Maybe the niece can lend you some of her clothes/blankets/ things that would smell like her/his home. Then if you can like the previous replier wrote, give him an isolated room, he could at least get used to the new house smell, then meeting the other animals can come later on.

Good luck with Jack