Well, nicotine from cigarettes is far, far more addictive than marijuana. Ironically, people typically might use cigarettes as a crutch to get off marijuana.
Why do you think pot is worse than cigarettes? Pot does not kill 500,000 people a year like tobacco does. The intoxicating and psychological effects are no worse than alcohol.
And what will it solve to replace the pot with some other drug or chemical? That gets him nowhere and could actually be worse. Sure, you could walk into a Dr. and walk out with a prozac or welbutrin script, but do you really want to see your brother go down that road the rest of his life.
If the guy can quit cigarettes, than he should be able to quit weed, too. Especially since marijuana is not highly addictive, at least not in the sense of other drugs. You do not get physically ill, the way heroin addicts and the like do when going into withdraw.
Some people get into trouble with the law, or get new jobs that require drug testing. In most cases they just stop getting high. Plain and simple. Just like anything else, you have to want to quit.
OK, to actually answer your question, there are perfectly legal "weed alternatives" that you can order off the internet. You see them in the back of magazines. They look kind of like marijuana, and you smoke it. But it doesn't get you high