Needs advice of mums?

i have a 3 month old baby who is constantly hungry. he drinks 9 oz bottles every 2-4 hours and is still hungry after them i;ve changed his milk several times and he's on hungry baby milk but nothing is working. he used to sleep throu but now he's up all nite hungry for milk. i've been told a bit of a baby rusk or a small breakfast with his milk will fill him up but i dont want him goin fat. i know he'll work it off when he crawlin n walkin but has any mums ever tried this? lots of people r tellin me to try it but extra advice would be great...he was 8lb 3oz at birth so he a big boy.