New born question ..

So I have a new born almost a month (in 3days) and I don't feed him breast milk no more just formula, I feed him 2oz every 2-3 hours but I'm starting to feed him 3oz and he doesn't seem to get full because he will cry for milk in an hour. (I try everything before I feed him more milk) Doctors say they gave a small tummy so I don't know..I don't want to over feed him and get him sick 😞