Marina,
Just my 5 cents....
As you already heard a lot depends on what the treatment is going to be, surgery/chemo/rad.

1) Assuming he receives radiation treatment, is a PEG tube going to be inevitable in his case?
I had one and I hated it! But I still recommend getting one. Some RO's absolutely require this. It does make things easier (hydration and nutrition) when you need it. Clearly, if you can eat normally do so as long as possible! On the other hand, if it turns out that you need a PEG, it would be very bad to insert this after a month of radiation when everything in inflamed!! (they go through your mouth)

2) Will he lose his voice during radiation treatment? If not how impaired will his voice be?
Expect changes... and this depends on the the radiation field and as always on the individual. There was a time when I used note pads (and hand signals) to communicate.

3) He is a realtor and self-employed. He seems to think he will still be able to work a bit during the treatment. Is this in any way realistic or is he in denial?
Again there are big variations... but do not count on it. Initially he should be able to do some work if all goes well. Later on he probably would not have the stamina to drive a lot and show houses (Assuming problem 2 is not an issue). Also if he is on Morphine/Oxycodone or other stuff not much work might get done.

M