To start with, it would be best to let him know how you feel, give him more motivation to stop. He could join a support group, as this has been relatively successful for some people I know. You could simply take the cigarettes away from him (and attempt to prevent him from getting more), as intervening is necessary at this point. His behavior will change, but take this as something that will get better in time.
P.S. I thought this was actually about your brother for a minute there, so that's why my answer is phrased like that.