I don't know about other nipples compatible with the Medela bottles, but I do know that the lactation consultants at my hospital DO recommend the xxxxxxxxx since it has a wide nipple that is similar to the breast. Back in 2001, xxxxxx used to also make a wide nurser system (I used it with my daughter), but they no longer have one from what I have seen in the stores. I know this doesn't answer your specific question, but I hope it helps. The gagging sounds more like he is reacting to the flow being too fast. Can you try a slow-flow nipple, even if it isn't one compatible with your Medela bottles? Maybe the stage 1 or stage 2 (which ever one you are using) is too fast of a flow. They can also try a side-lying feeding position, that way the milk pools in the cheek instead of at the back of his mouth where he may becomed overwhelmed with the fluid. In the cheek babies are better able to figure out what to do with the milk. I hope I helped.
I hope I have helped! :-) -AMB
