Being an executive producer doesn't mean that he's necessarily investing anything in it. The title is most likely so that the production can pay him what his usual rate which they might not be able to afford to pay him as simply the lead actor, It's done a lot in TV productions when they either can't afford to give an actor a raise of if they can't afford them at their normal rate, how this exactly works I don't know, but I'm guessing that there's a separate pot of money in the budget for cast and for producers. In addition to extra money, it also gives the actor a little more say in what goes on in the production, more so than actors normally get.