What ways are available?
1. Brutal honesty. (If he is under mind control he'll switch off from you immediately).
2. Subtle questioning. (Will he smell a rat?)
3. Withdraw from the 'religious' side of the conversation. Leaving him to it.........for now.....forever.
....what have I missed?
All depends on how well you know this geyser and whether you'd get DF'd if he rats on you. I'll wager he's being 'encouraged'(TM) by elders or who he 'studies'(TM) with.
Beware you are now an MDA like me (Mentally diseased apostate). Whereas I appreciate you want to rescue him, it can be a bit of a fine line to walk.