Recommend stop. Collaborative health maintenance is critical to bf's health. There are guidelines and recommendations as to who should undergo testing/screening. This may help: http://www.Cancer.Net/publications-and-resources/what-know-ascos-guidelines/what-know-accp-and-asco-guideline-lung-cancer-screening/recommendations-lung-cancer-screening.
If he is over 55. Has at least 35 pack year history, eligible for ct screening. Simpler cheaper tests do not work, no good early detection for younger folks. Smokers are addicts. They cannot quit unless they want. They often get angy when you call attention to their addiction and bad habit.