I don't know about the Zantac...no experience with that. You could call your doc to ask about that. Doesn't sound like a food allergy to me...
My experience with green poo was because of too much foremilk. Do you do breast compression to really empty the breast? The foremilk goes through faster, so it's less absorbed, they have more gas, explosive green poo, that sort of thing. I would suggest trying the breast compression to make sure he's getting the fatty stuff at the end of the feed. I also doubled up my feeds for a couple of days (fed with the same breast 2 feeds in a row if single side feeding). I hesitate to suggest that to you without knowing about your supply etc., but you could consult a lactation consultant. That's all I can think of!
