My brother wants to smoke weed? HELP...I Love my brother and I don't want him hurt :/?

Okay..my brother is a good guy. He has never got into fights, doing well at school, has loads of friends and is really happy. he told me tonight that he wants to smoke weed. I asked him why and he just said it would be fun and it doesn't hurt you. I am sure it does :/ How can I help him, and try and convince him not to..Does any one have any reasons, or major health problems that come from smoking weed? Or some good arguments? I really need some help, becuause smoking weed is bad for you, isnt it? Thanks for any help you can give me xxxxxx