It is very unlikely that you can get pregnant while nursing.
Even if you don’t want to get pregnant, you should wean your son from nursing. By nursing him this late, you are teaching him that food is supposed to comfort him, and that is a road to food addictions and obesity.
There is no perfect age difference between children. This is something that you have to determine for your self. It sounds to me that you are getting over attached to your son, and second child may be good for you. If you over attach yourself to your son you will not let him grow up to be independent.
What ever you decide, I whish you all the best.