My son is 5 and whenever I upgraded him to a different drinking cup, I simply "lost" the old one- I actually threw them away so they could not be found by accident or be available for my caving in! I lamented with him that we could not find it, this did involve participating in a search and rescue effort for the lost cup. So, then you can either run to the store to pick out a new one with him to get a big boy cup, or have the replacement on hand. Trust me the desire to drink milk or juice will win out over the stucture, color, or straw content of the new cup. It may take a day of a grumpy & whiney kid, but he will not starve or dehydrate in that small period- he will just be REALLY annoying for a day. Worked everytime and helped me teach him to keep track of his stuff(a cool side effect)- he had a habit of taking his cup(or toys) and leaving it somewhere and forgetting where he left it. Good luck!