How do I get a dog to walk?

I have been walking these two wonderful pit bulls for about 2 months now. Up until now, everything has gone great. I walk them about two times a week. On a walk a few weeks ago, the bigger of the two, Rocket, refused to cross the street. This happened a few more times where he would refuse to walk past a certain distance I guess, so I told the owner. She said she had never had this issue before. It has been getting worse, and now it has gotten to the point where Rocket will not leave the immediate vicinity of his owner's house (i.e. Six feet away from the backyard or front door). I have now tried using treats and kneeling on the ground to coax him to come along, but it doesn't work. It is impossible to take him for a walk. What should I do? Apparently when the husband is home but not the wife, Rocket just sits outside waiting for her. I assume that he is suffering from separation anxiety but I would really like to be able to walk him, and I don't know what to do.