Sounds like a milk protein intolerance to me, I had to deal with it with my youngest. She never had formula as I breast fed her, but she was constantly pooping (minimum of 5 times a day) and had eczema so I went totally dairy free and it fixed the problem. It takes at least 2 weeks to know if it is the problem after going off dairy totally, as the milk protein has to get fully out of their system. There is no test to do other than an elimination diet. So taking formula with dairy in it will not solve anything. It is something they can grow out of, usually by 18 months old - though my daughter can only tolerate yogurt and cheese, not cow's milk now at 21 months old.

There can also be a problem with soy as it has a similar protein to the milk protein. My daughter drinks soy milk now okay, but I avoided it before. So if the mom wants to continue formula but is concerned about a milk intolerance she going to have to get some very expensive formula that is dairy and soy free.

I personally wouldn't just go by "what the doctor" says, as I got no advice or support from my pediatrician about the intolerance, I had to figure everything out myself and do lots of research.

I know he isn't your child, but I think it is good to know about the milk intolerence stuff. I can imagine it being very frustrating, especially dealing with all the pooping. Been there, done that!!!

I would do as other have said and just put it in a cup when he is with you, if he doesn't drink it all you could just tell mom he wasn't interested.