The only advice I can give is to stay on him. During my treatment there were times that eating, drinking, or even sitting up seemed like the hardest things in the world. And I know that when my wife tried to get me to eat or drink I know that there were times I got frustrated and was short or even mean about it. But without some prodding from her I know I would have ended up doing even worse.

I agree with checking about getting some extra fluid with treatment or in between. Dehydration is a vicious cycle; when you get dehydrated you feel terrible, and it's really hard to catch up with just drinking because of how bad you feel. The PEG is a last resort, but some people definitely need it to get through the treatment.