Ok - I'm assuming (bad to do, I know - but trying to figure things out from what's posted) you're putting the leash down someplace outside, with treats, and walking away (that's excellent). And he's outside, too. I'm also assuming he's fine with you petting him? That's one big question I have - does he like you petting him when the leash isn't involved? And does he like being with your other dog? There could be all kinds of things going on here. Maybe he's terrified of the leash, maybe something in that fenced area of your yard where you take him terrified him, maybe your other dog terrified him, maybe .. maybe .. maybe. So - many questions to be answered, first. IF - he's fine with you, and fine with your other dog, and fine with the yard, and fine with the leash as long as it's on the ground, then .... maybe sit down with a bunch of treats, and the leash by you (on the ground) and treat for coming around the leash. Be really patient. Wait him out. Then touch the leash and treat for coming around, and keep upping the ante over time - it may take days and days, and many many sessions. Baby steps. BUT - gotta find out answers to those other questions first.