What Causes Acid Reflux Immediately On Taking Food In A Child?

12 year old nephew, very active, very healthy, great diet is now having gastric issues. fine then he will eat and get upset stomach (crapmy and slight nausea) so he wont want to eat, very gassy. then it seems to go completely away and he is ravenous. He had GERD as an infant. he is also a competetive climber, chess club jazz sax, 8th grade Spanish and math.