i would never ever do this. if you want to call him by your last name, you can totally do that and give him a different legal first name. what about simon (last name), called (last name)?
imagine, 10 years from now, if you did call him (name name) and he finds out you had other options that wouldn’t have stuck him with two of the exact same name that led to bullying and confusion for his whole life 
i can’t imagine someone ever wanting two of the same name unless it was a stage name or something along those lines.