I mean no offence, but personally, it's not that brilliant a name, in my opinion.
If you really want to keep the name, just ignore him.
If he's doing it purposely, he'll drop the name eventually, otherwise, he won't cause enough problems for you to have a similar name.
Just ensure the people you want to know you, know the difference.
If you want to make it slightly more clearer, capitalize your name.
"Add Man" sticks out from 'addman' more than "add man", if you understand.
There's less of a chance for you two to be confused, if your names are more different. And, you can make it different without changing it a lot.
"Add Man"
"Add+Man"
"Add-Man"
Any of those will stop you from being confused with the other person.
