It may be possible to confirm family relationships by comparing test results with John 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 67 markers, haplogroup J-L70, FTDNA kit #55676
Mitochondrial DNA test-takers in the direct maternal line:
Family Tree DNA mtDNA Test Full Sequence, haplogroup C1, FTDNA kit #55676