Another thought is if the sibling data is confirmed by the current 23andMe and AncestryDNA results (meaning no adoptions or NPEs are in play), the matches to either sibling should be approximately the same (give or take a fraction of inheritance deviation). So AncestryDNA is kinda covered with the brother's results.
However, Family Tree DNA is also having a Father's Day sale, and a combined Family Finder autosomal test and basic 37-marker yDNA test is bundled at 20% off. If the brother were to buy that (before the end of the day, when the sale ends) that would get the autosomal results in the database of the other of the Big 3, plus would get a Y-chromosome test for a view of the paternal father-to-son-to-son lineage that's distinct from the autosomal DNA. Too, FTDNA is the only one of the Big 3 that keeps the sample in storage, available for future tests or upgrades without shipping another kit for a new sample. Mine has been on file there since 2004 and I've ordered about 10 different tests against it since then.