Should my smoker bf be tested for lung cancer! How do I bring this up?

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.
Stop smoking! The national lung screening trial has demonstrated that people who are aged 55-74, heavy smokers, or quit within 15 years have increased survival if they undergo screening for lung cancer with a low dose chest ct scan.
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.