6. In 2016, a father of two minor children in Cuyahoga county, Ohio, filed a claim to determine custody of the children. At the pretrial, the father informed the magistrate he wanted to be named a legal custodian because he paid child support. The magistrate appointed a guardian ad litem, a third party that considers the best interests of the children, to work with the couple and come to a resolution. On the day of the the trial, the mother and father informed the court they had settled the matter. The mother and father signed an agreement that was approved by the guardian ad litem. The agreement detailed that the children would live with the mother, subject to the father's visitation rights as set out in the agreement. The agreement was incorporated into the magistrate's decision, with the magistrate designating the mother as the legal custodian and residential parent. The father appealed the decision, asserting that because of a unilateral error on his part, where he thought he was signing a shared parenting agreement, the judgment should be overturned. Do you think the father has grounds to assert a unilateral mistake? What kind of evidence would the father need to successfully make his case? [IN RE JW, 2017 Ohio 8486 (2017).]

Please provide detailed answers and references. Thank you!