I think he should go to therapy(or back to it). He obviously has some more issues he needs to get out about his father leaving and his mother throwing a new man in his face is not going to help the situation at all.
Now the porno stuff, I think that's pretty normal for a 13 yo boy to be curious and looking at it. but i would have his mom try and limit his internet usage and possible block sights like that(by setting parental controls on his user name)
It would be best for this Mom to set stiff rules about the internet and put a block up, he shouldnt have been able to access it if she'd have had it in place. The PC needs to be in a public room not where he can have access to anything without supervision, and less of the internet ,Sports and games are better for children than them being fobbed off out of the way to watch TV or Use the PC at will.
in regards to the porn watching, instead of confronting him, put a block on the computer when he's not home, then the next time he tries to access it he obviously wont be able to. He will straight away realise that someone know he's looking up porn and will save the embarrassment of having to confront him. in saying that, it should be constantly monitored and if the porn returnes, then it's time to have a chat.
I feel you could be the best person for your grandson to talk to right now. The fact he's just entering adolescence and the recent breakup of his parents is going to have a tremendous impact on him. Spend alot of time with him, this will show him that you care about him and soon he will realise he can talk to you about things that are happening in his life. Family is important and i think their the people he needs to be around now :)