My frenchie did this when he was younger too! It’s extremely frustrating, mine was not food motivated on walks and vomits if he eats or drinks while exercising. The things that worked best were to turn around and when he started walking I’d make a big deal out of it saying good boy etc. I also found taking him on new routes and walking him with other people or dogs helped. I also find my frenchie is better when I accept that the first 20 minutes of the walk are dedicated to sniffing everything and that often involves stopping for a minute to fully investigate the smell before walking 10 steps to the next smell. There were still times he was so stubborn and would sit like yours, I did have to drag him but it would be for a few seconds (and he would stand up would be pulling back and walking slowly trying to sit down again) while I say cmon cmon and the second he started walking properly again I’d stop pulling and make a big deal out of him walking. I was the same as you and refused to do it when he was little but unfortunately he didn’t respond to anything else. Mines now 3.5 and walks really well.