Smoking is highly addictive and if he has been smoking for a long time, quitting is no easy matter. I think the advice of having his doctor talk to him is a good idea. Having worked with a lot of teenagers who are getting themselves hooked. I found it is really hard to get people to change. unless they want to. On the other hand if he is really beginning to have serious health issues, a wake up call from his doctor might help. You might also look for some smoking cessation programs, so that you have some positive suggestions if he sees the light. Many of these are actually covered by health insurance.

Sometimes a medical professional has a lot more clout than we do as wives.
My husband is not a smoker now. He quit when he met me 10 years ago, since I would not get seriously involved with him otherwise but in other matters, I find that I seldom get results by nagging him. Most of the time people do not make behavioral changes unless they want to. The key is to get him to see that it is a good idea for him too. Then maybe you could work together to figure out a reward system that would provide positive reinforcement. Being supportive is really important. I wish you well in this