STD/Invitation addressing question

So FH has this friend that everyone calls by his first two initials...lets say....JC. NO ONE ever calls him by his first name. Even though no one calls him by his first name I'm still supposed to address his STD/invitation as:
Mr. John C. Doe

NOT Mr. JC Doe, right?

FH doesnt agree with me but what do guys know!?!?

i addressed everything with proper names. the only thing i deviated on was the invitation inner envelope.

ex: for our aunts & uncles, i addressed the inner envelope as Aunt Diane & Uncle Kenny, instead of as Diane & Kenneth.