Hi Mary...
John is fortunate to have you on his side and I am glad he found relief with the Rentidine. Hopefully it will be a good long term solution.
I wanted to just add to the pile here and let you know that your husband is not alone with the heartburn. I too developed it about mid-treatment.
Although I think it may have started from the Cisplat, more and more I think it is from the radiation now.
I still have DAILY bouts of heartburn, and I used to almost never ever get it before.
Usually in the mid to late afternoon, and my diet has zero impact.
The only thing that works (worked) for me long-term was liquid Zantec. You might look into it as an alternate. It is easy to take via the PEG and offers nearly immediate relief, even for slight upset (sour) stomache in some cases.
Since I am not a Dr, I recommend that you and/or John discuss this and remember that there is no such thing as a stupid question of them.
He doesn't NEED to suffer ALL the effects to get the affects of treatment. If he is sick to his stomache, has heartburn, dry mouth or other side effects, be sure he let's his care team know since they can mitigate many of them
Good luck and please keep us posted.