+1 on not dragging him. Especially on footpaths it will abrade/cut his paw-pads.
We had/have this refusal problem, ours hated the collar from the outset, then hated the harness. Then hated cars. Treats and promises of going to the pub or back home are the only things that work, well that and me hiding behind a tree until he relents, but sometimes he just sits down far behind me, and I then secretly wish to punt him over the hedge.
He somehow knows exactly where halfway point of any given walk is, and then happily trots the second half back to home/car. Hilarious (sob)
Could be so many things – good luck getting to the bottom of it, but don’t force the issue in any stressful way, it will make things so much worse/more ingrained. He needs to know that 1. he’s safe, and that 2. he is required to walk – you are the pack leader after all.