There is no "right" to be angry. Angry is how one feels and all feelings aren't good, bad, right or wrong; they just are.

People doing things that are your responsibility behind your back implies they believe you are incompetent and that they have no respect for you.

I would tell him you want the games back or repurchased by X date, remind him they are his son's games, not his and give the consequence that if they are not, you are going to call the man and get it done yourself. That gives him an opportunity to step up to the plate. He knows where you are and what choices he can make relative to this problem you are having with him, to resolve it.