Wish him luck, oops, "good-fortune" and let him carry-on with what he plans to do.
As JW's, it was engrained in us that it's our responsibility to save everyone else. Although as humans , we wish to protect others from harm but unless the other person is open minded and will look at and accept facts when given, they will oppose whatever information is provided. That is usually OK in most circumstances but in the JW world, brain washing and blind allegence to the organization results in adherents reporting others for having views contrary to official policy. Someone that hasn't been attending and even those who live a lifestyle which is contrary to WT teachings, does not mean they no longer hold the WT as 'The Truth'.
If and when he is ready to question the validity of what is presented from the WT, he will then seek his own answers to his questions, wherever those answers may come from. As you are presently fading, you aren't recognized as taking a stand against the organization so in likelihood, he will not confide in you any doubts he may have if he has any. Just as you can't trust him to keep confidential any conversation you have with him, he also can't trust you for the same reasons.
If you do decide to share with him your reasons for questioning what you have been taught, it cannot be in writing or in a conversation that occurs with anyone else present.