If you are aware of his background you'd know that he indeed apparently started to call himself Boris - in his first term at Eton in 1977. Not sure he was in a position to further his political career at that stage beyond education...certainly doubt calling himself Boris would further it anyhow tbh
Sent from my SM-G935F using Tapatalk
So you're saying he couldn't by that time be ambitious and be doing things to further his own ambitions, be they academic/economic/journalistic/political and be creating two personas to help in his quest? As I say his close friends apparently call him Al or Alexander. That Acuri woman says she referred to him as 'Alex the Great'.
It's not to do with the benefit of any particular name (chances are it was just the first to come to mind as it's his middle name), it is the mask it provides him as a persona he puts forward to the public.
If you want to go psuedo-psychological it means he could also use it to disassociate himself from his worst behaviour (DID/MPD). Like Gollum and Smeagol in LOTR. This next bit is pure speculation but I know my nan used to use my mum's full name when she was unhappy with her or telling her off when she was little (and me). So she associated that name with naughtiness or trouble. Apparently this was common among her friends too. So if something similar happened to Alexander (who is roughly the similar generation) Boris could be the naughty little boy that gets in trouble. Alexander is the good boy.