Neighbor (investor) dispute re: shared retaining wall

[Mike Sag]

When I bought my duplex 3 years ago I noticed that retaining wall separating my neighbor's lot from my property was showing signs of bulging and incorrect drainage. I finally was able to get a hold of the owner and showed him the damage. At that point he disagreed it was of concern and we (reluctantly on my part) agreed to monitor the progress and then reconvene. This year I saw that damage did progress and called him again, we met and this time he just flat out refuses to deal with it. I would gladly offer to split the cost with him, because I know rebuilding the wall is a lot more expensive than improving landscape and correcting drainage from his lot, but again he just says "Not my problem, the wall is on your lot". I checked for CC&Rs and there is none, old property. Anything I can do short of blighting the bullet and doing it myself?

Thanks in advance,