[Opinions] Re: New nickname
Some of my friends have nicknames. Only about half are actually derived from their names.
Nicknames aren't supposed to be legit. So no "legitimate form of nicknaming". They don't have to be planned out. So it's completely fine, and I don't think it's outdated (my friends and I are around 14).
And a flaw in your reasoning: He can still be "Teddy B". Theodore isn't really uncommon/nonexistant, and some do go by Teddy.