I came close to quiting a couple of times myself, but thought of my own "Daddy's Little Girls" and had to do everything I could to fight this disease. It was hell, but it was worth it to get to see my grandchildren grow up.
Pain management is critical. A very wise oncology nurse told me that my cancer wouldn't be cured any faster by my trying to tough it out. I took the pain meds and it helped immensely with my quality of life through the remainder of treatments. I was on the Duragesic patches for the baseline pain, supplemented with liquid morphine just prior to eating (i didn't have a PEG).
The pain meds do have a downside,in that they will frequently lead to terrible constipation... PAINFUL constipation, so they need to be on top of that.
I too was a stubborn old "non-complainer". I got over it, and accepted any form of help that was offered.
I wish there were some kind of medal I could award my wife for putting up with me and pushing me through treatment and recovery, but she seems satisfied to still have me around six years later.
Go to his appointments with him if you can, and make sure his doctors know what's going on. It is hell right now, but in the long run, you (and he) will be glad he saw it through to completion.