I agree with the others.

Put a word to it - it's always handy to have a dog that wees on command anyway, especially when it's late at night and cold and you want to go to bed and your dog is ambling around the garden, pausing only to sniff the air from time to time

And rather than hanging around with a jam jar, maybe put a cloth under where he usually wees at home and then put it in a carrier bag to take out on a walk? Once he's "been" once at a particular spot (hopefully it will soak through the cloth and scent mark the ground so you don't have to spend the rest of your life carrying smelly cloths around), that should get him going