You could try shouting and banging tin trays when he looks like he is going to venture over there, and scare him back to his own garden.
Alternatively, get a (very trusted) friend to drive their car up the road when he's around with lots of revving and horn blowing. If the first doesn't stop him the second should at least make him a little more wary.
Although I live in only a small lane, this has worked on my two and as soon as they see a car they dive to safety at whichever side of the road is nearest.
Good luck