I think that all the suggestions on here so far are illegal. Minors cannot (legally) join the 'regular' poker sites
, although I have encountered self-confessed underage players on some of them.
So far as Zynga, or whatever it is called, is concerned, it matters little what their age limit is, surely local laws still apply regarding age for gambling/gaming.
The only thing I can think of is, as someone suggested, letting him watch as you play whilst you explain the whats and whys. Occasionally asking him what to do but doing the playing yourself.
Even then you might fall foul of the 'contributing to the delinquency of a minor' type of laws, not legally competent to say for certain.
Whatever you do, don't teach him too well though or in a few years you will find a very difficult opponent.