[fostermom]

I agree that there might be some medical issue with this pup...please get him checked out. If you decide you are willing to work with this pup here are
a couple of thoughts on working with this pup...
1. try leaving some kind of short leash attached (or cut the loop for the hand in a regular leash so that he doesn't get a foot caught up in that)...that way it would be easier to catch him and you won't be chasing him around trying to attach a leash
2. it would be best if you can get him to come to you rather than you approaching him...kneel down, trying to position your body so it is sideways, and do not look directly at him...extend your hand with a treat in it and let him come to you to get it..do NOT walk up to him, bend over him, look directly at him, or attempt to pet him on top the head as a shy dog will interpret all of these as threatening
3. you might want to work with him with a clicker (clicker training is great for building confidence in a shy dog)..whenever he does a desired behavior, click and treat (even if you have to toss the treat to him). I would reward ANY appropriate behavior and ignore shy behavior. A rescuer I know was fostering an abused and extremely fearful dog...she clicked and rewarded for anything as small the dog holding her head erect, any eye contact initiated by the dog, wagging it's tail...any NORMAL behavior.
4. whenever he is behaving nervous or shy, do not pet, reassure him as that reinforces the behavior.
5. sometimes having a social dog around helps the shy dog model more appropriate behavior.

Best of luck...