Hi Gigi, what DNA test did he take -- Y-DNA, mtDNA or auDNA? My guess, since you mention ethnicity, is that it was autosomal. If that is the case, you should upload his test results to Gedmatch. It's free to do so. If you haven't set up a profile for your brother on Wikitree, do that and set the privacy to "Private with Public Biography and Family Tree".
Gedmatch will assign an ID number to your brother's kit. Enter the information from both the site where he tested (i.e. ancestryDNA, 23andMe, or FamilyTreeDNA) as well as his Gedmatch ID #.
Gedmatch will show you matches. Start with the people who share the most/largest segments with your brother. The larger the amount/segment, the closer in time your common ancestor will have lived. So if there are some that he shares 20+cM with, start with those (by sending an email to the person administering the kit). If there aren't any that large, use smaller, but never smaller than 7cM.
You also can look for matches on the site where he tested. It's best if you can upload a gedcom file to both Gedmatch and the testing site.