It may be possible to confirm family relationships by comparing test results with Allan 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 37 markers, haplogroup R-M269, FTDNA kit #291107
Mitochondrial DNA test-takers in the direct maternal line:
Family Tree DNA mtDNA Test HVR1, haplogroup H, FTDNA kit #407400
It is likely that these autosomal DNA test-takers will share some percentage of DNA with Allan:
AncestryDNA, GEDmatch RG6372435[compare], Ancestry member allanharlthomas