Show him examples of what most good players do to try and fake you out.
Show him how to counter various mindgames (There are plenty, but most of them have a similar concept) and how to 'predict' what is about to be done.
If you practice with him, and show him individual mindgames, then test him with all of them mixed together, I think he can learn to counter very easily.
I have no competition.
The best competition I have is a fellow Fox player, but he's nowhere near my level (semi pro I would say, given techniques I apply and how I apply them compared to pro videos and such).
I work with a lv.9 Fox, and apply the most thinking I can to anything I find.
Oh, wait.
You mean BRAWL.
Well, since it's a lot slower than Melee, it's easier to see where you're going.
But I still fake people out with Ike and land a killer fsmash, usmash, or fair.
It all depends on how he plays.
I guess just show him different characters, their moves, and how to counter them.
Simple, but it will take time to get used to noticing when someone's smash has started, how long it can charge, and when the smash will end.
Fox's usmash and Ike's fsmash are relatively hard to dodge (Well, not really Ike's, but he does have awkward timing on it), but over time I have found that I can see Fox's smashes coming 1/4th of a mile away. (Means it's still hard, even though I have sharpened skills)
Remind him:
He's not always going to win.
I learned that from some pro Snake player on with Anyone, and my friend who plays Kirby and DK.
Also remind him that characters have favored moves, so teach him what moves are used most, and get him used to dodging and countering.
After some time, he should have a firm grasp of the concept, and be one of the best kids out there.