1 year toddler suddenly reject milk

My boy just turned 1 year old. He had been drinking NaN Optipro 2 and now 1, we added in NaN Optipro 3. We mixed them at first to let him get used to the taste. He was drinking fine a few days ago but these few days, he just simply refused to drink. His daily formula milk intake is now less than 400ml. It’s kind of worrying. Any advice from mummies how I should make him drink more milk?