Wow... Im not sure what to say. Im horrified at the situation. And mostly Im sorry... this is a terrible thing for you, your family and him to go through. He may not understand palliative care but I am sure he knows on a base level that the treatment will likely not save him (though Im the type of person who believes in the never say never policy... because someone with stage one cancer can die from it and someone with stage four can survive)... Perhaps he is in denial or just content though likely in pain. Because of his health history (lung mets)/alcohol abuse Im almost afraid to suggest a second opinion but my head and heart tell me to tell you to fight...If you do seek a second opinion and they tell you the same thing then I would be inclined to say stop the rads if he's still doing them. It will just make things more painful in the long run... Chemo might help shrink the tumors. But rads is brutal on the healthiest of people.
About the mucous... very common with rads I drooled like a dog... a big dog actually... in fact I could have given marmaduke a run for his money. The radiation irritates the lining of the mouth. He must be in pain... I just can't imagine. Protein powder mixed in with his feeds will help him with any healing that needs to be done. Does he have magic mouthwash? There are good creams available for his skin. Are they certain that what is in his lungs is in fact cancer... (sorry these are all things that are running through my head!)
I guess what this all comes down to is a decision has to be made. Fight it... if possible with everything you have. Or accept that he is terminal (very hard to do with loved ones) and let him enjoy his last days... and franky if that means letting him have his beer then I would. What I do believe is that in order to make a decision like that you need to be informed. It sounds like a lot of information has not been shared. I would ask the dr. what the purpose of the rads is. Is it truly beneficial. Are they working towards trying to heal him or do the palliative thing, and if it is palliative then what about quality of life? Rads can ruin that quickly.
I really just wish I could give you a hug, because I am just saddened by all this and it sounds like you are an amazing and caring daughter in law( I know how it feels to have some one in your life who is terminal - my mom died of lung cancer at 59 - I am an only child.. I know she knew deep down what her prognosis was but we never actually discussed it. However we were always aware and informed about what was going on, why things were being done etc..)I'm so sorry. But you do definitely need to be given all the information. I personally believe that in order to give consent to treatment you need to informed... if there are pieces of the puzzle missing then that's just irresponsible medicine. So talk to your doctor straight up. And mostly remember that doctors normal people like you and me... they are educated and informed but not gods.

take care...