I'd be just a tad concerned that he might have a flip-flop about the Witnesses and want to go back some day. But you know him and we don't, so I have no idea how firmly he believes it is not the truth. If you're not 100% sure that he is 100% that it's not the truth, if you think there might be some shred of doubt remaining in him ("What if they're right and Armageddon is coming?"), then I suggest asking him to make some kind of definitive declaration. In other words, ask him to declare, "I am confident that Witnesses don't have the truth, and here is why: ...". Don't help him. Even challenge him a little if you feel like it ("Yes, but did the Witnesses predict 1914?", etc.).
Also, make sure that if his family started shunning him, he wouldn't fake being a Witness just to get back in their good graces (does he make a living and live independently, or might he still need financial/emotional support from family?). His "faking it" won't necessarily harm your future relationship, but the danger is that he will be spending time under the influence of the congregation. If he's not rock-solid in his beliefs, they could suck him in. But as I said, maybe you know him well enough to know that this can't possibly happen.