Girls good day, could you please tell me, for a 2-month-old baby, in t…

Girls good day, could you please tell me, for a 2-month-old baby, in the first month the increase was 700 grams, in the second 300 we are on IV, the doctor said that this is very little, that it may be due to the heart and that we urgently need to see a cardiologist, maybe someone has encountered this, eats also not normal, and he holds out somehow every 3 hours, before I give him to eat, he doesn't eat, in a day he eats 400-500 grams, we feed Nutrilon Premium, maybe porridge should be changed