There can be a sudden onset of anything because of the illness and because of any over the counter meds as well. Yes, I have experience! I was diagnosed with Ulcerative Colitis when I was 11 years old and was not treated properly and lived with it for years. It all starts with the fact that there is NO acid in the stomach to digest food. Meds and illnesses can get the chemical balance of a body off. That's what happened to me. Acid reflux and GERD as it is sometimes described is caused by a lack of acid. The stomach is supposed to be acidic to break the food down so that it will digest and keep moving through his little system. When there is no acid it sits in the stomach and rots...
When cleaning out your refrigerator, have you ever opened a piece of tupperware when it had something really old in it? It spits at you because of the gases that have built up. That's what happens with reflux..the gases in the stomach from the undigested food have no where to go so they push against the esophageal valve...and it burns because the esophagus is alkaline not acidic...Being on Mylanta may give temporary relief but it will rebound and come back with a vengeance.
Probiotics, prebiotics, a good absorbable multivitamin, raw fruits and vegetables and possibly a fiber supplement. His system needs to get back to where it is supposed to be before it does any further damage. If your doctor does not think this is important now, I would consult with a different one or go to a ND instead of an MD. They have a better handle on nutrition.