Sorry he is having such a bad time! My mucositis was awful as well. Towards the end cream of wheat with lots of butter (no salt) was pretty much the only thing I could eat aside from vanilla ice cream w/milk. Luckily ensure did the trick in keeping the weight on though.
Unfortunately I still felt pretty awful for about 1 month after treatment. Is he on pain medicine? I was on a ton of pain medicine (175 mcg fentanyl patch and dilaudid or hydrocodone for breakthrough pain). If he isn't on pain meds he should definitely get on them until his pain level gets better. For me I think everything peaked (pain/discomfort wise) at my last week of radiation (i only had 6 weeks), then it stayed around that level for about 2 weeks. I think that if he can still get enough Ensure down though, he should be able to avoid the feeding tube. However, if he does get on it I am sure he will only need it for a few weeks - it may be nice for him not to always have to worry about getting calories (that always stressed me out).

Sorry I probably wasn't too much comfort, I hope he starts feeling better soon!