R.,
Congratulations on breastfeeding for so long! It is great that you want to continue to do this wonderful, healthy gift for your son!
How much breastmilk do you want available for him? There's no right or wrong amount! If you want to continue to breast feed, you will want to have him nurse definitely during those times you want to continue (waking, napping, sleeping at night).
I found I was feeding my son too much when he first started eating - he didn't have room for the milk he wanted. My supply started to drop off, and it hasn't come back the way it was since. He is still very much wanting to nurse, but very disappointed when the milk runs out.
You could breastfeed him for his entire first year and he would not be the worse for it - a little low in iron perhaps, but this is not the case with every woman. So, if you are really curious, call La Leche League or visit Dr. Jack Newman's site:
http://breastfeedingonline.com/newman.shtml
Good luck!
M.