a) by leaving a substantial amount behind for him just like the rest of the children; you are showing him unconditional love. He may be touched by this and want to return. However a child over his teen years who has gone completely off, it is more than an 80% chance of him never returning.
b)a couple who does not observe taharat hamishpachah causes their kids to go off the derech.. once the kids have grown and chosen their path it inevitably becomes clear to the rest of the world how stringent the parents were when it came to this halacha…