This is a very easy one, if you leave your place of employment you forego any perks of the job and have to give back the company car etc and Archie’s father has chosen to leave the well known firm called the Royal Family.
If you are not ordained as a Vicar you do not have the right to use the title of Vicar, or if you are not in the Police force and chose to impersonate the Chief Constable you can be arrested, God forbid that someone who is not a qualified brain surgeon should be allowed to poke around willy-nilly in some poor unsuspecting soul’s head, you cannot have the title if you are not qualified for the job.
Unfortunately Archie’s father has chosen to leave the Royal Family and distance himself and his family from them and to live in America allegedly because of the pressure of being a member of the Royal Family and the detrimental effect it has had on his mental health.
It seems to me that you can’t have it both ways, you are either in or out, if you want your son to have the title of Prince then you have to return to Great Britain and take up where you left off as a working Royal.
Harry has chosen to leave the job, so with that decision he now has to give up the company car and any perks of the job which he or members of his family may have been entitled to like any other employee, he has made his bed, now he has to lay in it.