Well, I'm not an expereienced unschooler, but first of all, you don't teach your child everything. You teach them what they ask to be taught, and you teach them how to teach themselves. You can unschool for as long as it works for the both of you. I know a woman who "graduated" her unschooled daughter last year, at 17. I asked her how she knew she was "graduated", and she said bascially cause she was the right age! I think that unschooling will be something that just develops gradually. Your son will learn to walk and talk and deal socially with other people, and all of that stuff... you can basically consider yourself unschooling right now, it's just different stuff that he's learning!
I hope more experienced people will chime in to share their experiences, and perhaps expand on what I've said, because I'd like to hear more about unschooling too!