Why do i keep owing Apple money?

For further context, this is not me personally but my brother who is 14. Every month or so we give him money so he can spent it on stream apps (Wrestling, Sports, Twitch). I believe we put a credit card on his account then wait for him to buy said streaming services. After 24 hours we block said card (via the banking app) because we have had problems with him spending large amount of money. Anyway, after a few months he will want to download a free app and he cant download it because we find out we owe money to apple... Why do these apps give him another month access even though they cant guarantee a payment???? / How can we stop owing them money? (Is it because he choose a renewable subscription instead of just a month?)