Thanks for the full description. My first thought is that you are doing an excellent job meeting your son's needs for nurture and comfort, despite these challenges. So please be sure to give yourself credit for your patience and hard work, even if it feels like you can never keep him happy.
You don't mention his poops -- how often do they happen and what do they look like in his diaper?
How frequently is he nursing? (For example, in the past 24 hours, roughly how many times did you put him to breast?)
You mentioned that you are now nursing two times in a row on the same breast. When did you start doing that? Have you seen any changes, in the baby or in yourself?
Besides the Zantac, has he had any other medications? Any formula, ever?
Like you, at this point I'm thinking oversupply more than dairy issues.