[ADVICE] My 12 week old has stopped walking on the leash after doing perfectly well for two weeks prior. Need some tips on how to-recondition him.

I had great success walking him on the leash for good distances. Everyone was happy. Until yesterday, he just refused to walk on the leash. He would walk beside me dragging the leash, no problem, but as soon as I picked it up, he's sit down and wouldn't budge. No matter how patient I was, calling him to me etc he wouldnt take more than one step before sitting right back down.

Super frustrating. I feel like I have accidentally trained him that when he's on the leash he can simply drag it around freely... and protests the moment i take charge of the leash.

He's a dog that naturally follows me and will stay close, but distractions - people and other dogs, get him super excited. Besides that, I walk on roads. He is definitely no spooked by anything, because as soon as I let the leash go, he simply follows me again.

Any advice for re-training him to walk on the leash? I use clicker and treats for most training and he's been amazing, but I didnt have much success yesterday as he would only take a step to me and then sit right back down.

Was it wrong of me to put the leash on and then let him walk freely? Has he decided he is now the master of the leash etc?

help please :)