rescued abused golden

Question
I am fostering a puppy mill stud, dept caged for six years then sent to auction--our rescue group bought him and others.  He is so terrified, that after 2 errks in loving foster care, he is still cowering in a corner, cannot make eye contact.   Can be taken out on leash for potty, but terrified and back to the corner.  He makes no sounds, no movements, if he see a person, he runs to new corner.  Could he have some underlying medical condition?  Can this type of anxiety be overcome--any advise?

Answer
If he came from a puppymill, then yes, it IS highly likely that he has some sort of medical condition, but his behavior is that of a dog that has never been exposed to anything, or received any kind of socialization. It's going to be slow going with him, and it's not going to be easy, but if he is young, there is a chance that he will learn to trust humans. I've never had to deal with such severe anxiety before, so I would suggest working with a behavioralist.