A:Answer Hi, Anonymous - any of the prepaid Boost cards will work. However, depending on the plan/options you (or your brother) chose for his account you may need more than one.
If you signed up for an account that is $45/month, then by getting a $50 card, you are overpaying for that month (no refund).
My son is at a $45/month level right now. Occassionally I'll purchase a $50 or $55 card to add to his account. This basically just means I've overpaid - and I can go with a smaller amount next month or later.
Hope this helps!