I think this is a pretty common puppy thing. It’s hard to think that a puppy wouldn’t understand that the leash ties him to you, but it makes sense. He just needs to figure out that when he’s on the leash he’s sorta stuck. If he is just trying to walk away, wait patiently until he stops fighting, then call him back to you and reward him when he comes. Don’t pull back, just hold firm or give a slight tug. It may take a little while at first, think of it as a toddler throwing a temper tantrum. He’ll figure it out. If he REALLY starts to panick, give him a little slack till he calms down, THEN call him back to you and reward him. He just needs to figure out that the leash is a solid thing that he can’t just walk away from. Every puppy I’ve ever had went through this. I dog sat for a teacup chihuahua a while ago and didn’t realize he’d NEVER been on a leash. He was almost 2 year old. I went to take the dogs for a walk and he FLIPPED when he hit the end of the leash. It took us the better part of an hour to get around the block because I had to stop and wait for him to stop throwing his temper tantrums before we could continue walking. However, by the time we were half way done, he was walking very well, with just a hitch every now and then. Just be patient.