Since milk is his food currency right now too bad mom won't let you give him a small cup of milk IF he finishes the food on his plate and is still thirsty. His body is craving something from the dairy and it is feeling like it isn't getting enough.

Is there a chance she took him off the milk because all he wanted to do was drink and not eat. That too is a very common thing for kids and one of the reasons when I have a group like that I do give them water for lunch and milk with snacks is so that they can't just drink the milk and eat the starch to tide them over to the next yummy snack.