It's a cult and if you want to stop him being drawn in you need to tread carefully. They know how to reel people in and slowly cut them off from whatever 'outside' support network they have until their whole life is inside the church. They will then bleed them dry for every last penny they've got and make them feel guilty they can't contribute more. Before long they'll be recruiting new members.
The early stages of scientology often result in apparently positive changes to people as they gain confidence, a friendly support network, and are made to feel loved and valued. They learn some pop psychology from the courses they are paying for, some coping techniques for dealing with their anxieties, and they start to believe there is no upper limit to the benefits that can be unlocked. This is the trick, convincing them if they keep paying they can gain superhuman powers of insight, confidence, ability to take control of any person in any situation, etc.
You need to be the person that he can talk to without being made to feel opposed or belittled, which obviously means avoiding reminding him that he's been stupid enough to join a cult. Gentle but critical questions about what he's been up to, without judgement. Letting him see for himself that the gains he's been making won't continue forever, and making sure he knows you'll be there to support him if he decides to break away, without an "I told you so".
I wish I could say more but this is all I know from reading up on similar cases, as it's not actually affected me personally.