I feel for you, too, Pip. And I don't blame you for worrying about the boy. But I agree with the others that there's not much that you can do. Whether GS's behavior is simply a result of his parents' style of childrearing, a medical/emotional problem or computer addiction, it's up to his parents to deal, for the most part.
Any attempt to interfere is likely to get you distanced and even cut off from this DS' family. Still, if I thought it might help GS in any way, I might suggest that you take that risk. But, trust me, it won't - b/c his parents won't listen and will, most likely, just defend him if you try to discipline him, yourself.
I agree with others that you and DH can have your own rules in your home (and maybe you do?). But you may find them difficult to enforce with this GS, especially if his parents are there and countermand you. If they're not there but disagree with the consequences you dole out (time out or whatever), then you may hear about it or they might decide not to let him come there again w/o one of them being present. But it's certainly worth a try, IMO.
In the main, sorry to say, you and DH may have to avoid being around GS, at least till he's older. You can't do much about GS, but you can do something to shield yourselves from his awful behavior.
Of course, that will mean not being around DS or GD as much either (sigh). It's a choice you two will have to make. (Maybe you can have a "just girls" day with GD and, perhaps, any other GDs you have, but that's all I can think of to get around that.)
I just hope trisher is right and GS straightens out as he gets older. I've heard of that happening, too, so it's a possibility, I believe.