People have very different attitudes to swearing. Some people just don't see an issue, whilst others are very upset by it. Maybe this chap is in the former camp, and is not being aggressive in his choice of language, just has a different attitude from yours?
I would be a lot ore concerned about the shouting (although not entirely comfortable with the swearing,I would try not to judge). Shouting is aggressive, and I wouldn't like to think of an 8 year old, who can't really answer back, having to take being shouted at by someone bigger than him.
I don't know that there is much that you can do, though - particularly as all you have to go on is hearsay, and as you are not allowed to see your grandson's mother (your DIL?). Is there a reason for that?
Does he come to visit you at all? If you got a chance to speak directly to him, without putting words into his mouth, you might find that it puts your mind at rest, or that he tells you something more concrete that you can act on in some way. Alternatively, you could contact his school and ask them to keep an eye on him, but I would do that only as a last resort.