Also, saving money could mean a lot of things at 16. Maybe for your birthday, Christmas, or other holidays you are given cash and you save that money. You do side work, which could be anything from a legit job (not sure the legal age to work there) or doing favors like booking award tickets for people.
At the end of the day... if he starts claiming that he is self-funded and writes all his flights are award flights with miles he's purchased, flying on premium long haul cash tickets, etc... then yeah... I think you'd have a right to question him.
Being in Australia and limited to 4 credit cards... that would tell me he's not flying on sign-up bonuses. I'm also guessing there MS opportunities are limited as well. I would assume that most of his travel is through award flights where he has purchased miles (something he stated in his CNN interview).