This is am action that would be based upon the cause of action: fraud.  It is my understanding that the statute of limitations for fraud(or the time within which you are permitted to bring the action) is three years.  So if you knew that they did this in 2003 then you had until 2006 to sue them for the fraud.  As for claiming that it was a forgery, you did something to affirm the loan by paying it in 2010.  Claiming now that it the agreement was forged, even if you could, would be difficult.  I honestly can not see any way that you can recover against them at this point in time.  I think that you should now concentrate on doing what it takes to either refinance the loan to your benefit.  Or maybe pay it off with his intended inheritance and have the attorney that prepares your Will state why specifically in the Will so that he can not contest it.  Also put in a no contest clause.  Good luck to you.