I recommend Red Box -- especially if you have the rules up through level 36.
Some things to consider:
1. He will have fond memories of his first gaming experiences, just like you do, and it may to some degree shape his preferences and his approach to gaming. I suggest picking a game that reinforces the style and values you wish to impart to your offspring. And there's nothing wrong with giving him the same game YOU started with (unless it was F.A.T.A.L. or something
).
2. Kids have no trouble with imagination. As an adult is tempting to pick a game with lots of options, because it seems more versatile to you. Your son will have no problems with versatility, and even BD&D will be putty in his hands.
3. At that age kids have a simple view of the world. Morality is black and white, mosters are to be killed, and 36th level isn't just a theoretical limit it is a destiny. BD&D has a simple view of the world that perfectly matches that. A kid neither wants nor needs the depth or complexity or ambiguity that appeals to adults.
4. There's something epic about 36th level and the chance at immortality that just isn't the same in any other D&D. My son is fascinated (if not obsessed) with the concept. 3E/4E offer hints of that but it's much more complex, and there is so much fluff and options layered over it you have to study it to really see it. In BD&D the progression is stripped down to a very clean core that is immediately obvious and irresistable.
5. I gave my son my Basic book. He devoured it, then devoured the Expert book, then the entire Rules Cyclopedia. In no time flat he was designing dungeons and DMing for me, creating new character classes and house rules, all sorts of crazy stuff. IMO the Red Box is the most approachable version of the game ever written and also the most do-it-yourself version. It doesn't just let you make your own dungeon, it has a step by step tutorial.
As mentioned, I started my son with '81 Basic/Expert. We played in the Keep on the Borderlands countless times. It's worth nothing that BD&D is his favorite hands down, and he's played 3E, 4E, Fantasy Hero, GURPS, Fudge, and Savage Worlds. He likes FH and Fudge well enough, but oddly hates 3E and 4E, though he likes to browse the books. I had to twist his arm to get him to try 4E, thinking he'd like it, and to my surprise he hates it vehemently. (Frankly I suspect if I'd started him on 4E he'd hate BD&D just as passionately. 10-year-olds love are passionate and stubborn about their likes and dislikes! As I said think carefully before you leap.)
