my partner passed away from a lung disease, not the same one as your husband, but the deterioration was very much the same, so i can relate to what you are going through, when my partner was told he would need oxygen, he fought as long as he could before giving in to it, he did not give up on life, but did give up worrying about how much he drank (he never had smoked)... i don't have long to live I'm going to live it how i want.... i did try to encourage him to take it easy but after a few tries i realised the time we still had was valuable and decided not to waste it worrying or arguing about his drinking, the time we had was special, so many people do not have the opportunity to build memories, plan their funeral or say goodbye, we had all those and so too do you, make the most of the time you have together, because once he is gone you may have a long time before you see him again.
he will be going through so much at present, not only adjusting to life in the slower lane, but he will be pondering on the progression and the end and feeling like he has no control of his life anymore, probably be scared of it even if he doesn't admit it, he will be thinking about his past, the regrets and mistakes he made, and be worried how this is/will affect you among other things, so disaproving of his smoking is the last thing he needs right now. He will know that it may help him have a little longer and will decide himself if he wants to try giving up, but if like my man he decides he would rather go quicker than hang around struggling for every breath then you need to respect his decision no matter how much it breaks your heart, because it is one thing he still feels he has control of.
even when my partner was told he could not leave hospital until oxygen was installed at home he still fought against it, in all reality it does sound scary, but once it was installed it was notthat bad, though it did make his nose very sore. we still managed to go to the coast for holidays/weekends away as the oxygen was transportable, we had a concentrator which he used when we had electricity, we had large bottles of oxygen for the journey and a small refillable one for going out for a couple of hours. it did take a lot of organizing and i do recall screaming at his doctor on a number of occasions because he hadn't put the right number of bottles on his script!
so yes it is probably the hardest thing you have ever done, but you just need to be there, accept that he is in charge of his own destiny all you can do is help make his time happy, comfortable and as normal as you can.
my thoughts are with you both x