I didn't mean literally feed it like a living creature. [Why did you think that? Of course, a robot doesn't have to eat! That'd be such a messy, inconvenient design...]

If he doesn't get charged up or anything, maybe you should check for any... compartments or... like, those covered holes cars have that you put fuel in?

Or maybe leave him out in a sunny place. Maybe he's solar-powered, like one of those old-timey calculator.