Hi there, sorry to hear things aren't going too smoothly with this transition. Could you perhaps go even slower with the transition? Not sure how you are doing it, one ounce at a time, but perhaps go even slower?
I've heard other mom's go through similar experiences and the babies usually will, slowly, increase their intake on the new milk. Sometimes it can just take them awhile to adjust to it.
Rice milk is great if your lo is eating lots of fats and proteins, because the rice milk lacks in this area compared to soy/diary. But I know moms who add protein and fat to rice milk to make it work, I had to do that at a point as well.