Re: My dad is 81 and does not know he is dying of Lung cancer
First off, I'm very sorry for your father's current health and my prayers go out to him and yourself.
You're going to be a state of confusion and probably euphoria due to this, which is perfectly understandable as we love our parents and it's only natural to get worried over their health.
The reason you mentioned his speaking disorder is probably due to what his reaction might be if you told him, and how he wouldn't be able to express himself as normally as you would be able to in this kind of situation so he would freak out or faint. This might happen depending on his reaction to the news. I can't say for sure how he'll react since you obviously know him better than I, so it's up to you to decide if he should know or not.
My brother told me this story of his Japanese friend who's father was in the same health as yours right now and how his friend didn't tell his father he had it, leading to his survival and thus recovery from the illness - this could have been made up, but I am not sure. He said that this was the way of the Japanese, it was a tactic to save loved ones, and like I said, as for it's legitimacy, a quick Google search would probably be useful.
You love your father, you want to do everything you can for him, and you're now at the point of decisions and choice making - which is the most important stage of this process and also the conclusion of it.
I bid you farewell and all the jazz and take care.