Feliway is helpful for anxious cats. Here's a link:http://www.vpl.com/cgi-bin/vpl.cgi/1...ml?id=75vMFxXK
You can get a diffuser that spreads the odor, or just a spray bottle. Feliway gives a comforting odor to cats, that tells them this is a safe place. It is definitely worth a try.
There are also medications to help anxious cats, but as in humans, I don't think vets want to jump straight to medications. If you have a new cat, and it is sick and being pilled, that is reason enough to make it anxious.
After he feels better, and gets a chance to settle into the house, he may improve a lot. If he hides under the bed, and is afraid to come out to use the litterbox, unfortunately the answer is to put a litterbox under (or next to) the bed! And if you can catch him to pill him, he is not a feral cat.
Some cats are just shy or anxious. After he heals, and gets a chance to settle in, if he continues to be excessively shy, maybe the vet will medicate him. But although he may never be a total lovebug, if he becomes more comfortable with you he will be fine. As for avoiding visitors, many cats do that!