Hi, is he eating solids well? After 1 yo, I don't think you'd need to worry so much about the milk intake. Your breastmilk (even though little) still gives him a lot of nutrients. In addition, to the formula milk, you can also try to introduce different kind of milk such as almond milk or oat milk.
Also, is he taking the milk in a bottle? This is a good time to transition to straw cup or cup. The change from bottle to straw cup may excite him to drink more.