I'm frankly surprised you did as well as you did, really really think you should leave it unless there's other circumstances coming up, and try your best to make the time you spend with your son as good as possible for both of you. Please talk to a therapist for yourself about your expectations and how to handle your maturing child and this situation.
Really, I am with him, your child, as in that once he started going to a therapist, he began to be less enthused about telling you every little detail of his life and circumstances. "Mind your own business" from your child is a clue you may be over-asking.
You must've spent hours with him going over this business, detailing abuse for your court appearances and I bet he's tired of you talking about these constant unhappy subjects with him.
It is certainly not your business to trash the other parent constantly and his other living situation and that's what sounds to me like what is going on. Remember, you can't use any of this stuff again in a future custody case, so throw away your lists and quit inquiring so diligently.
Your job is to live responsibly while co parenting him, and set a good example. I agree with the question, what are you doing besides all this legal obsessiveness? If he is so hopelessly behind in his school work, you'll have half the time to see if you can get him enrolled in some enrichment programs and be more involved, so that he can be less behind.