I would ask his doctors to have a conversation with him about his smoking. There are Rx strength nicotine replacement therapies like inhalers and nose sprays that will get him over the withdrawls. Truth is that those that continue to smoke during and after treatments have very poor long term outcomes. He has to come to terms with this since his survival may weigh on it. If he will not listen to you, hopefully his doctors will put it to him in a manner that is non negociable. They may even be unaware that he is still smoking. You should have a private conversation about this with the doctors.