Let me ask you a question: If he truly loved you right now (and he also knew that you loved him), wouldn't he already be happy? And if all it took to get him off of drugs was being happy, wouldn't he already have quit them by now? I think you know the answer.
People turn to drugs for all sorts of reasons–including the ones your fiancé has mentioned. But one of the reasons why drugs are so dangerous (and why they are illegal) is because a person can very easily become addicted to them. And when that happens, it becomes almost impossible for a person to pull away without outside help. Drugs also can alter a person's personality or even do permanent damage to the brain.
Insist that your fiancé seek treatment for his drug problem if he won't quit–and if refuses, you should take it as a sign that he won't (or can't) quit once you're married. Some of the saddest letters I receive are from men and women whose marriage is being destroyed by drugs or alcohol.
Above all, urge your fiancé to turn to Christ and commit his life to Him. He needs Christ–and so do you. God loves you, and He doesn't want either one of you to wreck your future by following the wrong path. Jesus' promise is for you: "I have come that they may have life, and have it to the full" (John 10:10).