I suggest, while you still have a few days to "experiment", try putting a little bit of formula in with the breast milk. See how he reacts to it. I was about your age when I left my boys to go away for the weekend, and when I got back, my boobs were so swollen from not having my baby to nurse, so keep that in mind. You are concerned about your son, which is understandable, but think about yourself too. Your Mom is GRANDMA, she knows what she is doing, trust her, your babies will be fine. Go have fun, and try the mixture, if it still makes him sick, maybe try some Gerber oatmeal, it will thicken up the breastmilk, and he will get fuller! I hope it helps!!