I think the co-dependency literature has a lot of good stuff in it about healthy relationship and learning to deal with our limitations, even for people who aren’t “addicted” to anything.
I would not lay his difficulty in school at any particular pair of feet, at any rate. Your son might be having a hard time at school as a result of your divorce without it being a “cry” for anything that anyone else could possibly supply. Assessing blame is not only counter-productive. At this point in his life, it is probably impossible. Even if you knew, what would that change?
Think about how your divorce has affected you. There’s all this conflict, the break-up of your family, and you felt powerless to stop it. Your son is in a similar boat, excepting that he is even more powerless than you are and he gets to listen to your wife’s discontent more than you do. He can’t leave, he’s stuck with it.
Grieving, disengaging, or protesting the loss of one’s family can come out in school or work as a “bury myself in my work”, where you work harder and do more, but it can also express itself in an added difficulty in getting motivated. It can express itself in a reflex that wants to be as contrary as everyone else seems to be. Whatever the case, when you’re sad or mad, it can be hard to get going. The fact that you realize that you need to get going or that your reaction is primarily hurting you instead of anyone else doesn’t always change that. It doesn’t make the sadness or the anger go away. A pressure to perform when you’re hurting can make you even more resistant to do it. Refusing to perform, after all, is something your son can achieve, and it is a way to exert his independence from what his parents want. If your divorce was altogether independent from what he wanted, exerting his own will and making you live with it may feel kind of good to him, even if he’s not doing that consciously, and really really crummy at the same time. It doesn’t feel too good to exert your will when it does nothing in the end to address the problem you are powerless to address.
It may be that his grief and life disruption are going to take some time to get through, and it is going to be difficult for him to get motivated for school in the meantime. He’s going through a kind of a death, only it’s not really over. Your family hasn’t come to equilibrium yet. You’re still figuring out the shuffling of your roles.
Whenever you talk with him about his grades, express your concern primarily for him. “If you blow a year of high school, that can be a hard hole to get out of, but it is possible. I’m concerned about your grades, but I’m primarily concerned about you.” If he tells you that he erases your PMs, you can say, “Better that you erase them than that you want them and never get them. I’m a big boy, and I had more control over this situation than you do. If you’re pissed off at me, I’m going to have to live with it, because frankly, I can’t say that I wouldn’t feel the same. I’m still your dad, though, and you deserve to have me stick around in case you ever change your mind. You’re worth it, this is the pits, I wish you never had to go through it, but you’re going to be a fine man when you get through this, that is the main thing I’m trying to let you know–not that the divorce is good for you or teaches anything, but that you’re the kind of good guy that even a divorce can’t beat in the end.”
Or, as a friend of mine used to say, let him know that you think he’s “a floater, not a sinker.”