My 2 cents:
Lotion - Cetaphil or Cerave, lotion up as soon as his session is over, put it everywhere in the radiation field, and I mean EVERYWHERE, crack included, I did this in the dressing room right after the radiation session before going back to work
Cetaphil soap - you need something mild to wash with, this one comes in a bar, but it's really mild, don't use any type of regular soap in the area
Calmoseptine - this is a must during the shitting razor blades phase
Gloves - for the application of the calmoseptine for the above mentioned razor blades phase
Lotomil - a prescription anti-diarrheal, way better than immodium, ask early and have it on hand just in case.
Rinse & Repeat
How you can help him emotionally is a different story. My advice, treat him as your husband, not as a patient. Don't be his "caregiver", be his spouse. Be real, if you're afraid, admit it, tell him it sucks, just be there.
Good luck to him & you.