Kankani
Hello ..sorry about your Dad.
My husband had a stage IV tongue cancer, he had radiation and Chemo but no surgery.
My husband did not have the IMRT, I am now glad they did not, although it does appear that it saves more of the salivary glands. As his cancer was more advanced the field that was radiated was larger.
I find it unusual that no Chemo is planned for your dad, have they explained why this is?

Radiation will make your Dad very weak so make sure he tries to eat as much as possible, does he have a PEG and PORT?

Is he going to get amifostine before radiation treatments? This will help protect his salivary glands. It can make him very sick but he can get anti-nausea meds after the injection and before his radiotheraphy starts.
Hydration is very important especially during radiation he must drink LOTS.
I know this is a scary time for you but there is every chance your dad will come through just fine.
Anything else we can help with just ask.

Marica