At gunpoint.
On a serious note, tell him
"it's pretty much gmod with a lego-related style to it"
if they don't like that then the game's not for them.
but with less ragdolls.
Or try:
"it's internet legos"
If none of these work, it isn't a game for him.
Or try convincing him that you can shoot on it and make your own things, like RPG's and TDM's.