It may be possible to confirm family relationships by comparing test results with Jeff or other carriers of his ancestors' Y-chromosome or mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree:
Family Tree DNA Y-DNA Test 111 markers, haplogroup R-BY50713, FTDNA kit #B258103
Mitochondrial DNA test-takers in the direct maternal line:
Family Tree DNA mtDNA Test Full Sequence, haplogroup H1a3, FTDNA kit #B258103
It is likely that these autosomal DNA test-takers will share DNA with Jeff: