Elianne,
It is very important that your husband have an experienced treatment team and hopefully he does.

Under the circumstances, the team's decision to bring a psychiatrist on-board sounds very wise.

You will need to keep his doctors apprised of his tendency to use supplements, because some of these can degrade the ability of the radiation to kill the cancer cells.

Hopefully his team includes a nutritionist that he is seeing at least weekly. If not I suggest you ask for one. The nutritionist will help make sure his intake of calories and fluids is appropriate. FYI, I never exceed 2400 cal/day, but my minimum daily fluid intake was closer to 64 ounces AND all of this was diligently monitored by the nutritionist.

A break in treatment is not good because it can allow the remaining cancer to recover.

Radiation can not be redone. Our head and neck tissues can only tolerate around 72gy total (normally given as 2gy/day, 5 days a week for 7 weeks).

If your husband does not have a PEG tube, you should discuss getting one with his doctors. If your husband tolerates the liquid diet, it can greatly simplify nutrition and fluid intake during treatment and recovery and it is easy to keep track of the daily calories consumed (to maintain swallowing capability, he will need to keep consuming a little soft food and fluid daily by month, but not much).

I am a bit surprised that he had a radial neck dissection and so many lymph nodes removed - only one being positive. It seem more common these days to minimize node removal (selective neck dissection) based on exploratory and PET scans, or rely totally on chemo with neck radiation to kill the lymph node cancer.