clever: I think you have it.
They raised him like this, what can they expect? Irreligious folks don’t realize that the “one thing they hold dear” will be as meaningless as the rest of it to the next generation. I would say tefilla is your only option. If he is close to the one orthodox relative, he is the only one who can try to pull something off. You really can’t “trick” these people into religion. They think they know it all and just don’t want to hear anything.
That said, there are, of course, many “success” stories out there. Tefilla, and the right contacts are the way to go.