Sorry for the loss of your old friend.
I can think of a few reasons why your aunt would change his name and be overprotective of him. Usually some kind of family issue with hiding him from one of the parents. He may have found out when he was older that the other name was his real name and decided to go by that. If the birth certificate was never changed or it was a "family" adoption (un-wed underage mother) then you could see how it might have been difficult on him. I am guessing this took place in the 50's or 60's. These are just a few of the ideas that popped into my head (mainly from watching old episodes of unsolved mysteries.)
I would remember him for who he was, not the name he went by.
