I think my dog was abused before we found him, and that might be why, but now I want to walk him and he won’t walk!
You need to break the association that leash = bad. Start by getting to leash = normal.
You’re going to need to watch him while he has it on to make sure he doesn’t get stuck on anything and then get stressed out as a result, but just let him run around with a short one on for a while until this doesn’t bother him anymore. He’ll probably freak out at first, but there’s only so long a dog will let being put on a leash get in the way of something else interesting if opportunities arise.
Another possiblity is that he’s never seen a leash before, too. Young puppies happly take anything new like that as normal and ho-hum, but older puppies and adults often DO NOT. I got an adult dog once who’d not been mistreated, but just had never been leashed by the previous owner as that wasn’t important in those circumstances. . . she’d either stop dead in her tracks and dig in or try to throw herself around like a horse to see if that’d get it off. (This was after she’d finally gotten used to the idea of a collar; she threw fits over that, too.) A few weeks of letting her drag one around at least would get her to walk out the door with it on — then it was another few weeks to actually get her to go for a walk.