still puking- what could it be??

o posted in the GI section but i think it fits here too – okay ladies i’m posting for a friend on this one. Her son is 6 months
old he is a BIG happy baby all of 25 pounds. Since he was born he has
had issues spitting up. a couple weeks later is was throwing up. he had
a couple trips to the ER after chocking on spit up and stopping
breathing, but nothing ever came of it except the pedi put him on rice
cereal thickened bottles at 6 weeks. 1 tbsp per oz with a regular milk
based formula. i told her about MSPI but she insisted he did not have
it. and he continued to puke. switched to oatmeal less constipation but
still puked. The pedi put him on zantac 2 mls a day and regulan and
puking didn’t stop. after visiting a GI, a upper GI and a ph probe both
came back normal. they upped his regulan and he stopped puking, just
normal baby spit up every once in a while from 3 months to 5 months.
Now all this time she is telling me this stuff she said over and over
he dose not scream, he sleeps through the night, dose not arch or stick
his fingers in his mouth, he is not in pain he just pukes. (I was sooo
jealous- except for the puking part lol) Well fast forward to 4 months
he is doing solid foods, he eats everything under the sun and starts to
pukes again, the GI says its a case of overeating, she then insists
it’s not, i go over the food and formula with her, she dose consider
that she is feeding him too much and agrees to try what the GI
“quack”-she calls him- said to do, she always feeds him on a schedule
and dose not wait for his hunger cues. well he stops puking so much.
now for the past two months and this is the strange part…. he is
puking again. nothing has changed, eats the same stuff, he is maxed on
regulan and he is upright alll the time. he is in a walker or an
exersaucer whenever he is not sleeping. he also stays in his pack and
play and never pukes in there he dosen’t puke in his crib either. the
only difference is he is laying down on his belly or back. Her GI
insists she try previcid, but she refuses b/c they keep saying the ph
probe was normal, the upper GI was normal- why would he need previcid
if he is not in pain and it dosen’t stop puking. she goes through at
least 10-15 receiving blankets soaked through a day. she is going to
get a second GI opinion but she wanted me to post here to see if you
guys had any ideas. She wants to discontinue the zantac but i told her
she might want to keep it, it is probably the only thing keeping him
from being in pain. do you think that is a good idea? She is worried
about the long term damage that can be done from excessive puking. She
would be fine with the doctors answer being – he is fine he just throws
up- what would cause a baby to throw up standing up right only?