Register at GEDmatch.com and follow GEDmatch’s instructions to load his AncestryDNA results there. On your brother’s WikiTree profile, add his e-mail address in the e-mail address field. WikiTree will then e-mail him an invitation to join WikiTree. Have him accept that invitation. He should change his WikiTree privacy level from “Private” to “Private with Public Biography and Family Tree.” He should then add his GEDmatch ID to his DNA Tests page in WikiTree.
Then WikiTree will automatically associate his autosomal DNA and X chromosome with his relatives in WikiTree.