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, haplogroup Q-L663, FTDNA kit #839608 +
Mitochondrial DNA test-takers in the direct maternal line:
Family Tree DNA mtDNA Test Full Sequence, haplogroup J1c-C16261T-A18, FTDNA kit #839608
It is likely that these autosomal DNA test-takers will share DNA with John: