Have you checked out the "extended nursing" thread? There's a whole bunch of us who are happy continuing longer, and you might find it hits some chords with you too. I can tell you that most current guidelines (from health agencies) recommend nursing for a minimum of 2 years and then as long as both mother and baby wish to continue. There are many health benefits for extended nursing. Here's an interesting article on it (it talks about continuing past 6 months, but also about toddlers):http://www.bflrc.com/newman/breastfeeding/toddler.htm
I personally nursed my first for 18 months and stopped when I was pregnant for various reasons. My second I nursed until a little over 2 years when she basically self-weaned (with slight encouragement!). Prior to stopping, we were only nursing once a day in the morning for several months.
It's a very individual decision. If you do stop now, remember that you've given your son a wonderful start on life. But there's absolutely nothing wrong with continuing and many benefits. Trust your instincts and do what feels right for you and your lo!