I think the child in the video has been given a tremendous gift--that sense of
competency, that he can do something and do it well. This was something my own son learned from diving--and UW photography--at an early age. And I have dived with my son--alone--when he was about that age, although obviously we weren't spearfishing.
However, your comment makes me a bit uncomfortable because it smacks of hubris--and you know, when you exhibit hubris, the gods just want to smack you down.
Seriously, it seems a lot of responsibility to put on the slender shoulders of a boy who hasn't even reached puberty yet. He's not your equal--not physically, not intellectually, and not emotionally--and to expect him to be, to
demand that he be, is, IMHO, unwise--even if you did
train him yourself. What, does he always do everything just the way you've told him, because, if true, he would be the
most unusual 13-year old boy that ever lived. In a few years that can and
will all change--quite dramatically; one day he'll beat you at arm-wrestling and then he'll remind you of where you left the keys, etc. etc. Sunrise, sunset, and all that.