Was he OK when on the Neocate formula? If so, you have to ask yourself what is more important -- breastfeeding or the GI health of your kiddo. I am a mama who just finished bfeeding for 1 year my newest little 1 yr old kiddo so please don't think I am pro-formula....but 18 times in 1 day! I have seen 12 and I did have a foremilk/hindmilk imbalance and oversupply :) If you really want to keep it up, you might have to try eliminating the ricemilk and rice products (my daughter is on those b/c she has milk, egg, fish, and peanut allergies -- she is 2 yrs old) and doing the pro-biotics for you -- I take a dairy free one called kyo-dophilus you can get at GNC. It might not be Vegan though. It has instructions on it for infants but since your kiddo is nursing, it might be difficult to "sprinkle" some on food or juice (I used to put it in my kiddos bmilk bottles for daycare and I take it myself).
If you really want to know if it is a hindmilk/foremilk imbalance, you could double-pump - let the milk settle...make sure there is a good balance of cream on top and giving him bottles of it to see if he still reacts the same way. In the morning, I used to have an oversupply on one side that would result in 9 oz with NO CREAM and then it would start -- we all know a little baby is not drinking 9 oz at a time so that was his issue. If so, likely dietary and not imbalance.
Good luck with whatever you decide to do.