Heartburn is quite often a symptom of food sensitivities. Take note of when heartburn occurs. You may be able to connect the dots and find certain foods trigger heartburn.
Agreed, mint as well as caffeine lowers esophageal sphincter pressure, allowing acid to enter the esophagus, causing a burning sensation.

I also suffer(ed) from "heartburn". My doctor told me to stay upright and to actually avoid peppermint oil as it is a relaxant and can make the heartburn worse. My "heartburn" turned out to actually be my allergic reaction to foods so you might see if he is actually having an allergic reaction to something he ate.

Just apply 1drop peppermint oil to abdomen with a few drops of coconut oil. It wont burn.

You could go to a compounding pharmacy and get them to make it DF for you.

To find inactive ingredients in name brand drugs, go to:
www.rxlistcom
Longterm use of heartburn meds, particularly proton pump inhibitors such omeprazole (ie. Nexium), have side effects which include nutrient deficiencies that can cause bone thinning and other unwanted medical problems.