Eve,
He is suffering because he chooses to. Pain management (and the need for it with cancer patients) is well understood, there are NCCN guidelines how to deal with it and proper pain management will dramatically improve and shorten the healing process.

The worst is yet to come. I didn't hit bottom until 3-4 weeks post treatment. I can understand his anger and abusiveness. The treatment is very brutal but he must allow the doctors to manage his pain and get adequate nutrition and fluids or he won't make it. He may also need meds for depression, which commonly manifests itself as anger, particularly in men. Anti-anxiety medications may be helpful also.

You HAVE to force yourself to eat - fortunately this doesn't last forever - you CAN get past this and a month or so after he's done with radiation his taste buds should start returning, albeit slowly. This is where you have to get into the fight.

We have seen this happen before, mostly with elderly patients. This is the time to swallow the pride (not to bring religion into this but it should be no surprise that "pride" is the first of the 7 deadly sins) and listen to medical advice. Let his doctors know immediately the situation you are dealing with here.

Morphine is typically used as a fast acting "short term" opioid for breakthrough pain (it takes about 30 minutes to act - in pill form). Occasionally some people respond strangely to it and other meds may need to be tried. For ongoing, day to day, pain management he will need something like Duragesic which is a "long term" opioid in the form of a 72 hour time release patch. I had to take 2 morphine tablets just to be able to swallow water! (and that on top of the 75Mg of Fentanyl (Duragesic) that I was taking)

You HAVE to get his doctors, especially the MO, to enlighten him as to the benefits of pain management.

Like Karen said, your last resort may be hospitalization - maybe that's the convincer he needs to get with the program.

Tell your mom to be patient, get some respite support for her own sanity and he will feel SO BAD for the way he acted she can look forward to his making amends for quite some time once this is over (I bought a wife my wife a new BMW to make up for it ;-)