As you said, he wasn’t practicing. Is his family Catholic? Maybe not. Maybe not practicing either. How old is he? If he’s under (I think) 14 the law of abstinence doesn’t apply, if he’s under 18 he doesn’t have to fast. That may be one reason why he hasn’t been told.
People don’t usually advertise that they’re fasting or abstaining if they are doing so. And despite the huge crowds I saw at Ash Wednesday Mass, I didn’t see a single other person with ashes on outside the church - I think they tend to wipe them off.
So it may simply have never been something that was brought to his attention.