Revealing's up to him.
I'll only say his other username here isn't the one he posts at the other forums.
Why asking to surrender one account? Simple: it's regular forum standards. As any other forum we don't want multiple usernames either. It makes things confusing and creates murky waters for the mods & staff (who said who, how many warns does each user have, which has enough strikes to be banned, etc.) It is only reasonable to ask for one username only.
Others spotted having multiple usernames shouldn't be surprised when getting offered to keep only one. We want people to participate, yet not with multiple accounts.