Mrs W

Its sounds like the only viable option for his water intake is his PEG. So yes. Try it in small quantities at first and gradually increase per his tolerance. Water intake is very important.

Is he able to handle ice chips? That may ease some of the burning he feels in his mouth and also provide another source of water intake.

Maintaining swallowing should be a priority, but not at the cost of his caloric / water intake. (He can always work on the swallowing and trismus after treatment is completed.

Use the PEG tube as much as needed.

All the pain associated with the surgeries and subsequent treatment is unpleasant and downright nasty at times. But, its all necessary.

You need to take some time and care for yourself during this process as well.

Good Luck / Stay Strong