Attached is a part of the explanation of the abbreviations on the family cards of Rotterdam.
A R = Afzonderlijke register kaart ( a separate or individual /single) register card.
He was on June 24,1903 registered on this card and he came from the family card (G K). Why, is unclear.
But, when I look to his age (about 19 years) it is possible, that he went in the Dutch army for his duty. But, this is only my thought.