milk intake

hi mums. my lo is about 3 months and 7 days. usually he drinks 120ml every 3-3.5 hour. recently we realised that he spits up more often. we reduced to 90ml every 3 hours and no more spit up. question is, is it sufficient or am i under feeding him? once a while we tried to give 120ml, he can finish but seldom. but 90ml every 3 hours he will finish. last we checked his weight was about 5.3kg when he was 2 months old. i know about the formula calculation thing. he will drink less than 700ml per day. pee and poop is fine thought but i'm worried if i'm under feeding him and if it's not enough :(