Freezing on a leash can appear during the first weeks after adoption and is easy to make worse. Pulling adds pressure to a situation the dog may already find frightening or uncomfortable.
Rule out the boring explanations first
- Does the harness fit, and is it rubbing behind the elbows? A badly fitted harness stops a dog moving before fear does.
- Is this the first time he has worn one? Many shelter dogs have worn a slip lead and nothing else.
- Did the refusal start suddenly? Sudden refusal in a dog who was walking fine last week is a pain question, not a training question.
- Is the surface the problem? Some dogs will not cross metal grates, wet decking or shiny floors and are otherwise fine.
Build it in the house first
Put the harness on indoors and do nothing else with it. Feed him in it. Take it off. Repeat for a couple of days until it is unremarkable.
Then clip a light lead on and let it drag while he moves around the house, supervised so it cannot catch. Only after that does anyone pick the lead up, and the first thing that happens when they do is that he gets to walk toward something he wants, not away from where he is.
Outside, make the route short and the direction homeward
Some dogs are more willing to move toward home than away from it. If that is true for your dog and your dog can be transported safely, a very short return route may be easier than an outbound walk. Extend the distance gradually and stop if fear increases.
When he plants, stop pulling and wait. Standing still with a loose lead and no pressure resolves more freezes than anything you can do actively, because the pressure is usually part of what he is bracing against.
What makes this worse
Dragging, lifting and carrying a resisting dog forward, or using a lead correction to break the freeze, all add an unpleasant event to a situation the dog already finds unpleasant. Veterinary behavior guidance is consistent that aversive handling in a fearful dog risks increasing the fear rather than resolving it.
A dog who is not walking at all is also not getting exercise, which people tend to solve with longer attempts. Shorter and more frequent works better here, and sniffing in the garden is not nothing.
Once the dog moves willingly again, the same calm, loose leash habits from the guide to stopping leash pulling keep walks easy for both of you.