We rehabbed a very timid foster dog with crating & the umbilical method. Keep him in the crate except for a few hours a day when he’s attached to your belt by a leash, and make him follow you around without paying attention to him.
It allows him to learn to ignore the stimuli around him as he must pay attention to you at all times.
Then the crate gives him time to relax and feel safe in his den.
This should combined with long daily walks, so he doesn’t build up a lot of nervous tension.
Thats what my son is doing now. The dog likes him. Its the rest of us that he is terrified of and when I say terrified he is trying to jerk loose of the lead to get away from us.