Not enough formula

Je

So my son is 15.5lbs and based on the 2.5oz per pound he should eat 38.75oz a day. He eats 6oz five times a day and right before bed i give him cereal that you mix formula with and i give him 1 1/2 tbsp of that. Is this okay or should i try to feed him more? He also spits up a lot after eating.