Property Contract

A friend, who was taking care of his wife had been suffering from cancer. During such time they entrusted their son to have his lawyer friend draw up a contract to state should they pass the son gets the house and property. The son brings the contract to the father and dying mother to sign. "Sign here.." is all the son told them, trusting the son. After taking care of the wife 24/7 for 4 years the wife passed. The father decides he wishes to sell the house and property. He entrusts the son to get a realitor to sell it. The father then runs into the realitor who mentions the son will be getting a share. The father later confronts the son and thus finds that the contract he and his late wife signed was worded by the son's lawyer friend; If either parent were to pass the son inherits the house and property. Now they are in a dispute. The father and mother trusted the son who in turn pulled a fast one on the parents. Can this contract be challenged due to signed under duress?