A lawyer is the next step if the city fails me. I’ve never sued anybody in my life but at this point I would have no other choice. The old ways of violence or “knocking sense” into someone are unfortunately all but dead.at some point it might be worth a bit of time and money to have a lawyer get involved. Either to read the ownership agreement for the road and/or send a letter to the guy. Making him shell out some $ for a lawyer (or the thread of needing to) might sway him a bit. Even the idea of going to mediation.
perhaps it's not the route you want to go, but have you inquired if the city could take over the ownership of the road? Might be a political issue, but that'd at least allow for more clear guidelines.
And, as CHenry mentioned, i'm sure there is some kind of easement involved somewhere, so likely some group has a stake in the matter (even if the issue is maintaining the road to access other areas, like the highway easement)