I didn't catch how old your Dad is but it really doesn't matter. sounds like no one told him how bad it really gets. He should read this and understand that radiation of oral cancer and chemo is burning and poisoning but it is necessary. Please tell him to come read this forum so he can understand that what he is feeling was perfectly normal for many of us here. You do need to go with him and also make sure he understands that if the cancer comes back because he quit going for treatment, he will be praying for these days. Trust me, the second time is much much worse and he should want to know he did all he can to avoid it.
Good luck
Charm