Agree with Rose n Poos. Another thing... if you hook a leash to him (on harness preferably) and just leave it dangling without holding onto it, he is more likely to get used to the feel of it and start to see it as normal. Just let him hang out with a leash on around the house or yard.
My guess is that he has been punished on a leash, for example using aversives, such as prong collar, or even a flat collar but with yanks/pops on his leash . Try training him without a leash at all. My spoo has been trained to behave awesomely off leash.
When dog owners yank on their dog’s collar, they might be doing more damage than they could ever imagine; every time your dog yanks on its leash, it may be causing long-lasting negative health effects. Just imagine wearing a leash yourself, and now imagine having someone pull on it. When people...
My guess is that he is super intelligent and wants you to understand his dog language. Try studying Ian Dunbar
If you try the trailing leash, whether on collar or harness, use a shorter one and make sure he's always in view. You want to be sure the leash doesn't get caught on something and potentially scare or injure him.
Each approach is valid and his response to them should help clue you in to how to help you both move forward .
Maybe he ended up in rescue because he did not want his owners/walkers to dominate or control him, and he had been hurt or frightened by their doing so. I suggest trying the opposite, totally gentle approach Learn to listen to your dog, and speak his language, which is so very different than what humans think.