At the end of the day, all these things are his decision and many PALS choose no NIV and no G tube.
All you can do is give him the information and then advocate for the choices he wishes to make.
I don't believe as a CALS we can have any idea what this is like for our PALS, no matter how much time we spend with them or how much we do for, or with them. This is not something that you can fight and win.
It may be worth simply making the most of what time you have with him, letting him know you will advocate for him and acknowledge that the only control he has is to make these kinds of decisions.
My husband made many decisions I did not personally agree with, but I advocated for these things for him (he couldn't speak).
Would he have lived longer if I had made decisions for him, or insisted on talking him into decisions I wanted? Maybe, but then we can't say that for sure, it's not that simple.
Would my husband have enjoyed any extra time had I done that? I'm certain he would not have, as he would have felt he had no control and resented me doing that.
Be careful deciding things like 'he doesn't want to allow himself to be happy'. ALS is the worst thing I could imagine dealing with, and he is really newly diagnosed too.
Just love him and support him. Maybe he will decide differently, but maybe he will won't and all that will go far better if he knows his loved ones support whatever he chooses. I believe it will make the entire experience easier for your heart, and your family.