Just be a bit stern with him... but not overly demanding. Just lay it out how you see it but make sure he still understands that he has the option to decide for himself. If he is offended by that, then let him soak in it for a bit and revisit it later... if it continues, then sadly there's not much you can do other than ignore the issue and hopefully he doesn't press it on you.
I just look at the possible outcomes and what could be worse. Being there as a friend and convincing him it's not for him, then he leaves it early.... or not trying to talk him out of it and you watch as his life is affected negatively and/or goes into shambles.
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