poor milk intake...

Anak sy nk msuk 6 bualn.. Baru2 nie dye batuk dan selsema.. Selepas tu lately dye nmpk refuse minum susu dye.. Struggle utk bg dye minum susu.. Now he still drink 4 onz.. Tp dia kali minum baru hbis 4 onz tu.. Is it consider normal.. What should i do.. Should i change his milk to other brand.. I worry he will not get enough nutrition.. Can anyone help me.. Really need help