No, he may not. A contract or settlement may *only* be modified with either the consent of all parties (or at least their legal representatives: e.g someone who had legal guardianship or a POA) or by a court order (e.g. after a legal action in which a judge determines that there is some reason, such as fraud or a mutual mistake, that requires an alteration). No one party to a contract or agreement may unilaterally, or one-sidedly, alter it.