how to get him to drink/eat enough during the day so he sleeps at night

Our 6 month old boy has never enjoyed milk all that nuch, drinks only per necessity and averages 700ml of formula. He does have 3 small meals eq. of 50-100 ml (breakfast is blended oats, banana and apple, lunch and dinner is rice and lentils or potato broccoli, ie. some starch and veg combo. He also fights sleep a lot.