I don't think I've commented on your sitch before, but reading through this thread I can tell that your H is quite manipulative. If he had wanted his son to go to his old school, then why didn't he make sure you were living closer to the school while you were still a family. Surely, he wasn't expecting you or he to travel an hour to and from the school every day?
Just an aside ... make sure you're always happy and upbeat around S3. It seems he doesn't mind telling things out of the house, which is fine 'cause he's little and should be able to speak his mind. But, you need to protect him from your own pain and sadness.
I guess this would seem cheeky, but when your H talks about tradition, and wanting his son to go to his school like his dad did, then how about the tradition of his son growing up with both parents together (unless his parents are divorced, then he is sticking with tradition, I guess). No, best to say nothing ... I was just being weird.
I am sorry your H chose to introduce your S3 to OW. It seems so tacky to do so before you are divorced. I mean, what did he say? (Of course, I am surmising.) "Son, I'd like to introduce you to my slut ... I mean ... friend ... no, I mean girlfriend ... OW who enticed me away from your mother, and got me to break our family ... my homebreaker." Gosh! What an idiot? How did S3 take it? I'm sure your H was careful, if he has any sense.
Take care. I like the GAL activities you put forth at the beginning. I went back to uni, and it got me through a lot.
Me:57 H:52 M:28 Got another lawyer last year and filed. D35,S/D twins28,D22 EA4/04 End? Who knows? "Life is like a mirror. Smile at it and it smiles back at you." — Peace Pilgrim