Hello, M.!
I nursed both my children for 15 mos and was never able to pump more than 2-3 oz at a time. A child will be able to nurse more efficiently and pull more milk than you will ever be able to pump. As he continues to nurse during a session your body continues to produce, as well.
I'd guess his disinterest is more due to his growing skills and learning (aka playing) than a lack of supply. If he is taking in table foods more, that may also attribute to his lack of interest. I'm willing to bet it is a temporary situation though. It is almost unheard of for a child to wean himself that young and lack of milk supply is just as uncommon.
I'm not a doctor or lactation consultant, but I'd suggest munching down on some oatmeal cookies and heading to www.kellymom.com to see what the experts there say.
Lots of kids go through temporary nursing strikes for a multitude of reasons and your supply may be less now because he is nursing less, but it will increase/decrease at his speed.
Good luck and hang in there!!
J.