Thanks to you all for bringing up the common law aspect which made me find my answer. I think he could have filed at the time of the sale, and prove to a judge that I was unjust "enriched" by his contribution. Since I, as I mentioned, I paid for everything including the improvements, there would be no way in hell he could prove that claim.

Thanks again! And if anyone else happens to come across this question again (assuming its common law) there is the answer!