Actually, i think it would be a good thing to do with your son. The math skills you could teach him while playing a fun game may help him immensely in the future. I wouldn't worry to much about any negatives associated with poker or "gambling"...... for right now it's a game and could be a valuable learning tool. (ie. probabilites/odds/etc......)
I would not teach him online poker though..... I'm talking about sitting around the table with a deck of cards and some chips, having a family night....not sitting in front of a computer or with a laptop in his lap....
Online is gambling pure and simple but a home game can be just that..... a game.