Of course he's not a Marine. Slow your roll.
Send him a PM or something, calmly stating that claiming the title of Marine is not something to be taken lightly, and that you would appreciate it if he would not appropriate the deeds of others to pump himself up. And, if by change he's a private who got bounced out of the Corps after a month because of this, that, or the other, that he needs to represent the Corps better than posing with cammies and a white trash mustache.