Mom probably isn't even thinking about it as that is just the routine she has gotten into. She may be doing it just because you are giving it to him too. Get a 4 oz bottle or ask mom for one and limit how much he has in the bottle giving it only to him while sitting in the chair for snacks and meals. Then just send home what is not used. This means he will drink 12 of the 16 oz but you have extra in an emergency. Just tell mom that now that he is eating so many foods like the others he isn't as interested in so much milk.

Then start switching the bottle to a cup starting with the snacktimes.

I look at this issue two ways - where does my loyalty lie. Is it my job to perpetuate what the parents do or is it my job to do what is right by the child letting the child grow up and do age appropriate things. I decide when soothers, bottles, morning naps, use of scissors, playdough etc happen based on my experience and how the child acts at daycare.