Water bottle comes home almost full every single day

My son, class 5, takes a 750 ml bottle to school. Most days it comes back with maybe a quarter gone. He says there's no time, they can only drink in the break and he would rather play. His teacher says children can drink during class if they ask, which he apparently never does. What I've been doing is making him finish it when he gets home. Then he drinks half a litre in ten minutes and doesn't want dinner. A smaller bottle would at least get finished, but that feels like solving the wrong problem. I can't decide which is worse.