SS, adding my welcome. It sounds to me as though your father has come through well considering. Which is to say everything is relative in this nasty business, but the goal is being cancer-free which I'm hoping will be the case for him.
Yes, the few weeks after RT can be brutal. But they pass, and it's great that he has you to help him through it.
I actually think he did himself a great favor getting the PEG. I didn't and I wish I had because I found it nearly impossible even to drink water with all the mouth sores and dropped 30 pounds during treatment. And I was in no way overweight to begin.
Give him all our good vibes because everyone here is pulling for him. All the posters who've preceded me are veterans who always come up with excellent advice. Time, time, time... like everything else, healing from a tough treatment like his will take a bit more of it than we'd all like.
Courage!
oh and PS, you didn't mention whether or not they'd tested him for
HPV, and if he saw a dentist pre-RT to get fluouride trays and/or had his thyroid levels checked.