I have a 17 month old son and am still nursing him too. I would say not to rush it if he is not ready. The WHO and the APA both think any child that gets breastmilk for 2 years or more is a good thing. I think are society forces us to wean quicker than we really ought to, because they don't feel comfortable seeing an older child nurse, but somehow it is okay to see breast falling out of a woman's shirt on a billboard... Go figure.
Just remember you are giving him liquid Gold. Just to let you know how much your breastmilk is worth, I donated some of my breastmilk (274 oz) when my son was 6 months old to a milk bank. I was reading on line that is can go from anywhere from $2.75 an oz to $3.25 an oz, plus $.25 an oz to ship. Even some stomach cancer patients drink breast milk, because it is so gentle on their stomach and good for them. Pasturized cow's milk, can never be as good as your milk. He has all the time in the future when he is older to drink cow's milk. I hope this gives you a little support. Check out www.mercola.com website, he has some great health and holistic advice and articles. type in breastmilk and pasturized milk in the search box.