IMO this is a fine time to set a boundary with your H. You are not obligated to listen to ONE WORD about OW. And make it clear she is not to pick up your son from school. The audacity of your H to ask you for the SECOND time if OW can pick him up and then ask for yet another last minute schedule change because you won't comply to his demands. No. No. No.
He is trying to guilt you in to allowing OW to pick up your son and it needs to stop. If he is unable to pick up your son on his scheduled days then he needs to change his schedule or skip seeing his son. It is beyond cruel to bring up OW picking up your son again when you made it clear the first time you did not want that. Enough is enough.
IMO you need a good family law attny. 769.00 is way too much money to be owed when the money was spent on your son. Of course he should be paying for a portion of his school needs.
I'm telling you, get this stuff ASAP... once that baby is born money will be tight for them and then you will have a real battle on your hands.
Why have you not been following along with the procedure of sending out the invoices each month? How in the world did you think that would win him back? Being nice will not win WAS deep in affairs back. Set boundaries and tough nuts if he gets mad. You are not a human ATM machine and this is BS.