I voted yes, but I would ask the grandson first. "Do you want to hear this message from your father? It will explain why I have done what I have done. But, it's up to you. It's a very ugly message and if you don't want to hear it, you don't have to." Maybe find a way to record it on to something else and GIVE it to him to go listen, so he's alone when he does it. Having someone there watching him for a reaction seems cruel to me. This would be a private moment for him, I believe.
If he says no, drop it. If you can record it somewhere to save for later, do. He may change his mind. But, I wouldn't make him listen. I would offer. I think he deserves to know the truth. When I was 14, I had many lies told to me about my alcoholic father and the pain of those lies stayed with me a lot longer than the pain of the truth.
But, again, 14 is old enough to make this decision, I think. He may say no and that will be that. I'd still save it for later if it ever comes up again (and it might.)
I'm probably in the minority here, but that's how I feel about it. Good luck with your decision. Hugs and peace!
ZW