You really do need to work on detaching. It is not your problem if your H chooses not to read his mail. It is not YOUR fault he chooses not to read his mail. He had the information he simply chose not to read it. It is not up to you to tell him.
IMO the two of you need to sit down and have a talk about your son's schooling and summer activities in addition to the money he owes you. I would simply tell him: H, I feel it is important we have a discussion about son's upcoming summer activities, schooling and finances related to son. I am available <insert a few days here> and ask him what works for him.
DO NOT HAVE ANY R talks during this meeting. I know this is hurtful but if you don't start establishing something now as far as your son is concerned it will only get harder. As the pregnancy progresses more and more expenses will come up and time will be limited. Set your precedents now. You should not have in incur continued financial hardship because your H's mistress is pregnant.
