Sooner -

Has he given you a reason why he's not interested in eating? Nausea / pain?

How is he dealing with his diagnosis emotionally?

You need to talk with his doctors and get him onboard with his need to improve his situation. Weight loss during treatment is tough to stop, but intake of calories and water are incredibly important to his ability to get through the treatments.

Get him some help, now. If its physical his doctors have options / meds. If its emotional, get him to the right place. As soon as possible.

Use of the PEG is not defeat. Its a viable option for intake when the mouth and throat are no longer cooperating.

Good Luck