Hi Russell
The alias probably means nothing more sinister than
his dad died and his mum remarried a Mr Simpson.
The Henry might be his middle name or the name given him by his step-father.
May be step brother or step dad's name was George?
Also this might be why he did not know his birth year.
Look for death of his dad and mum's remarriage
or death of both parents and an informal adoption.
Or alternatively mum may have been single then married.
Everyone should note the Royals have lots of aliases.
Regards
pH