Why is my 4 mth old spitting up and vomiting so much?

My 4 mth old has been spitting up 7-10 x's per feeding since he was 2 wks old. the drs finally put him on Enfamil Nutrimagen and diag. him w/ acid reflux. they have him on zantac. but he is still doing it and it seems to get worst. he is underweight. he weighed 8 lbs 14 oz @ birth and now he only weighs 13 lbs. what should i do and what could it be? The peds don't seem to take me serious but this is serious to me. someone pls help. i wanna make my son better. and through all of this he is such a very happy baby.