90 seconds after he sells the place he will forget any promise to address the problem!
If he altered the topography and the water run off the proper solution could involve megabuck costs to regrade and properly drain the land!
A suit against the land (not just the owner) and a lis pendence might sure gum up his sale prospects.
Then again a nice clear readable sign on a post 1' inside your property line but facing his side, warning prospective buyers of possible major cost to address this defective retaining wall might produce faster results.
And depending on your state disclosure laws you might send both owner and his agent a copy of the city inspection report condemming his wall --they might be required to thus disclose it to buyers. .