decrease of milk intake

Since birth si baby ko feeding nya is 3oz every 2 hrs. Nung approaching kami ng 3 months, suddenly na decrease ? Plus he doesnt like the Nan HW. We had to change from plain due to his allergies. Now, 3 months and 2 weeks na sya in 24hrs minsan 18oz-20oz lang. Is that normal? :( Laki na tuloy ng pinayat nya.