I'm glad your scans are looking good, but I'm sorry to hear about your mother. Hopefully she'll recover from this w/little or no lasting effects.
About the children, why doesn't your xh contact the school and set up a meeting to discuss the grades? That would be the most logical thing to do. However, I think the issue w/the schooling/grades has a lot to do w/the stress that your child is under. He doesn't like going to xh's house and dealing w/the stepmother and her children. Your child may be suffering from a bit of depression as well. The next time he bashes you about not informing him of the grades, suggest he contact the school directly for an appointment and meet up w/the teachers to see what is going on. I might even include that you will be happy to attend the meeting as well. It's time to put this puppy to bed and stop bashing you.
vge, some of the MLCers remain angry for a very long time and some even stay that way forever. I do think he's trying to build a case against you to take the children away and not have to pay you any child support. He doesn't realize nor care that the children aren't happy w/him and the living arrangement. All he is looking at is the money.
You should contact your lawyer and advised him/her that he is bashing you to the children. He needs to be put on notice that this unacceptable behavior and is creating some issues w/the children, i.e., hence the grades falling down.
Don't put this off. The longer this goes on, the more likelihood your child will continue to lose interest in his school work and fail additional classes.
Good luck!
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.