Mite also b worth trying different formulas! Em was fine 4 a while on some formulas and then would lose weight. eventually we found out she is Lactose intolerant by giving her a tin of lactose free then switching back to normal. The paed said that the instant return of symptoms showed LI. She has challenges every now and then and every time she gets the same symptoms. now I am not saying ur DS is LI, all i am sayin is that sometimes different brands taste different and all of a sudden they will drink heaps. Also, How old is he?