"is eating" means, that he eats just in that moment. "eats" means, he is doing it on a regular basis. There is no simple way in German that reflect this kind of special meaning. To emphasize that something is happen while we talk about it, usually the word "gerade" is used: Er isst gerade (jetzt, genau jetzt, in diesem Moment) Zucker. That would indicate how unusual it is for him to do something like that - or at - or at the given time. Otherwise than that, "eats" and "is eating" are perfectly interchangeable.