Some think that blessed ashes should only be given to those capable of personal sin and so do not give them to children below the age of reason. This is a misunderstanding of the sacramental. The blessed ashes remind us of our mortality (e.g., the traditional words of imposition are "Remember, man, that you are dust and to dust you shall return") and of our common sinful condition, shared through original sin even by those incapable of personal sin (e.g., young children, the cognitively disabled).
If this should happen again on future Ash Wednesdays, politely ask that ashes be given to your son. If you get argument, you might point out that, "Today’s reading from Joel says that ‘children, even nursing infants’ (2:16) are to be included in the communal penance."