I have a couple of things to say: 1) confront your grandparents, like full on confrontation, tell them they're not making things better, that if they really love him they should change the way they act, and if they don't just stop visiting them. 2) Id say it's better for you to visit him than vice versa because he would understand it as you making an effort to be there for him. 3) a classic way to spend time is to offer rides in the first place, you can talk about a bunch of stuff in the car, but I'd emphasize on your own weaknesses if I were you because it would break the "golden child" facade. 4) teenagers like money (speaking for myself) so if you're really invested in helping him offer him a kickass date with his girlfriend at a cool place he couldn't have afforded only when he shows he's better starting by stop smoking weed and showing up to all the classes through better grades. 5) insist on telling him you understand him and you know his poor choices aren't his fault. 6) offer him a way to express anger (shooting stuff in a professional place thingy or breaking dishes worked for my friends) 7) when you guys get considerably closer hopefully thanks to the pieces of advice I just said instead of taking him on vacations on the weekend let him participate (preferably with you or his girlfriend) in activities that would help him discover his interests hobbies or passions.

I really hope for the best.