shy dog freezes on the leash

Question
We recently rescued a 5 month old Wheaten Terrier. He came from a hoarder's house. We slowly got him used to having a collar and leash on.  I can walk with him inside the house and inside the fenced in back yard. But when we go outside the yard, to the vet, or anywhere where there are cars, he freezes on his leash. He won't move, not even for treats.  We have been trying to desensitize him, by taking him to lots of different places. But the result is always the same, we have to pick him up. How do you help a scared dog learn to walk on his leash?  How do you boost his self confidence?

Answer
If you noticed, you said Vet. Going to the Vet is often a very scary experience.

I myself owned a Bernese Mountain Dog who would froze up quickly and wouldn't move, even peed on herself when we were to go somewhere.

Ideas:
Keep working with him in your front yard. Have him do obedience commands.
Move the work-outs outside your property but still directly in front of it, then venture out, day by day, little distances like the neighbor's yard, etc.

I would take my BMD to calm, settle places like a lake where nobody but us and some frogs hung out.
I put down a blanket, read a book, she'd munch on a bone.
Then we would go back home. Soon we were able to hang out in more populated areas and at the end she became a real social butterfly who couldn't wait to go somewhere.

PS: Obedience Training is so important when it comes to behavioral modification because it gives the dog something productive to do, which makes them feel good and well, happy, achieved dogs have it easier to overcome issues.