My answer depends on what you said to your grandson when he asked you not to tell.
If you promised him you would not tell his parents, then you cannot possibly do so without ruining the relationship you have with the boy.
If you said you would have to think things over, then the answer is not so straightforward.
Your son has earlier told you to mind your own business when you expressed concerns about him or his children, so quite honestly in your place I would hold my tongue.
If he discovers that the boy is watching porn? violent videos? and asks why on earth you did not mention that the child had sent you one by mistake, smile sweetly and say, "You have asked me yourself to mind my own business, so I did so in this instance even although I felt you should know about it."
If neither your son, nor the boy's mother knows that he watches these videos, they are not likely to be pleased if you bring the matter up.
If you haven't definitely promised your grandson that you will not talk to his father about this, I think you should have a quiet word with your grandson again, and try to help him realise why watching this sort of thing is not a good idea. He obviously realises that the adult world won't approve, or he would not have asked you not to tell.
You could ask the boy to promise you to stop watching this kind of video.
If he were my grandson I would prefer to remain on good terms with him, but to explain why you were shocked by the content of the video he sent you by mistake, and that you hope he no longer watches this sort of thing.