It may be possible to confirm family relationships by comparing test results with Brett or other carriers of his ancestors' Y-chromosome or mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree:
AncestryDNA Paternal Lineage (discontinued) 46 markers, haplogroup R1b, Ancestry member calpurnius +
Family Tree DNA Y-DNA Test 37 markers, haplogroup R-M269, FTDNA kit #157926
It is likely that these autosomal DNA test-takers will share some percentage of DNA with Brett:
AncestryDNA, GEDmatch A381998[compare], Ancestry member calpurnius
Family Tree DNA Family Finder, GEDmatch A381998[compare], Ancestry member calpurnius, FTDNA kit #157926