I would continue to put the milk in a sippy cup and if he drinks it fine, if not at least you tried. You could send home the sippy cup with mom so she can see how much he is drinking or use it for the evening so it's not wasted?

There are always going to be things parents do that we wouldn't but I think some things are not up to us and in this case, I think this is one of them. As long as he drinking water and eating meals I wouldn't stress about the formula. If mom is worried she can take him to the dr and the dr will be able to see that everything else in his diet is fine and there shouldn't be reason to worry.