Anne-Marie,
Your son's diagnosis and treatment sound fairly similar to mine (mine was 17 years ago at the age of 39), but it took me longer than 9 days into radiation to feel what he is feeling. However, not everyone reacts at the same rate, so what he's experiencing is not too surprising.
It's important that he get sufficient nutrition and hydration each day, so his medical team should be doing what's necessary to keep the pain under control. Also, if it becomes too difficult for him to eat and drink by mouth, he should probably have a PEG tube put in to enable him to keep up his intake. Is he being treated at a major cancer center? If he has an experienced head and neck cancer team, they should be anticipating these problems and helping him to be prepared for them.
Cathy