baby hungry all the time. help!

hi everyone. i have 2 weeks newborn. im pumping and he is on breastmilk only simetimes i give formula. he is hungry all the time. im giving him 2,5 or 2,75 ounces milk but he can go only 2 hours sometimes even 1,5 hour. doc said to strech that from 2h to 3h but i dont know how to do that he is screaming and my heart breaks so im givong him in 2 hours. plz ladies share ur experience. how to make his feeding 3h