It may be possible to confirm family relationships with John by comparing test results with other carriers of his ancestors' Y-chromosome or mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree:
James Edgar :
Family Tree DNA Y-DNA Test 111 markers, haplogroup I1a, Ysearch KB4J2, FTDNA kit #126416 +
Y-Chromosome Test 44 markers, haplogroup I1a, Ysearch KB4j2
Mitochondrial DNA test-takers in the direct maternal line: