Actually, a lone 4E D&D player character will be able to kick some serious ass (without getting into the whole argument.)
If you want to do that, I'd personally go with 4E. And actually, you could do a character as well. Think about it, two brothers or long-time friends doing various heroic quests to save the world.
The Monster Manual shows you which sort of monsters should be best for encounters depending on what you have in the party. Don't send skirmishers against a lone controller and don't send endless hordes of minions against a striker.
And while playing the games, maybe even show him the ropes of DMing if he never did it before. Make it less story-oriented, and have cliche quests of which most can be accomplished before the end of the session and put the monsters stats he is (or both of you are) fighting on the table where he can see it. Have him control the monsters during one fight at some point, and soon enough, both of you could be running each one a game at a turn.
I've got twin friends who did exactly this.
Also, re-skin everything. It doesn't matter who's stats they are, if this gnoll skirmisher is now a bandit, so be it. Monster manuals don't make good for stories where the opponents are low-level humans.
Anyway, that's my advice.