I just walk with the line loose, rolling it up and letting it out as he wanders and saying 'far enough' when he gets to the extent of it. 90% of the time he will sniff at poo and then return to me (without me saying anything) for a treat without attempting to eat it, if he does try and eat it I just say 'leave it' and he will. It was because I was having this amount of success that I decided to try him off the lead, but as soon as I did it was straight back to poo eating .
I would definitely try dropping the line and standing on it if he starts to stray past your "stop zone" and recall him. (I think)
