I was just going to suggest posting a sign but you're right, that might just cause more attitude about speeding.

I think I would just tell the guy again to do something about it or else. Remind him of the rules set when you agreed to allow him to use your road.

I've had to deal with so many neighbor problems over the past 10 years that I can really sympathize with you. All mine are child related - all the neighbors kids think my yard is the community playground. They used to even climb my fence around the back yard. I have no children so there is no reason for them to be here. Luckily at the moment all the kids immediately around me have moved - YEAH. Oh gee, I better knock on wood!