The pups have no clue what that strange thing on their neck is - so why not get a handful of yummy treats and call the pup to you and then let the pup go where ever his nose leads him (within reason - not out into the road or into a ditch!), all the while giving treats and make leash training as much fun as all the other stuff you're doing with him.
Put the leash on and let them drag it around out in the yard (or a fenced secure area) and make it FUN...
I know of folks with dogs that never see their leash or collar except for trips to the vet - you can imagine how much those dogs LOVE the leash and collar - NOT! They've commented to me about their dogs behavior with the collar & leash on and have asked us why our dogs don't act like that - well, DUH, we take them to places other than the vet, and we've TRAINED them!
