I feel so much for you, around how unfair and wrong it is that this total a-hole gets to act like this AND gets to take time away from you and S2. Yes, yes, yes, your child needs a strong relationship with his father. Yet. I 100% understand where you are coming from and it is the one place where the rage really hits me when I think about my sitch and the possibility that the end will mean I wake up to an empty house half of my days, no warm snuggly mornings.
My only thought for you is to focus on the long game-- getting the best deal for you and S2 out of this as possible. Document his every time being late or blowing off a weekend because he has something better to do. S2 comes home with a full diaper? Record it. I do want to agree with DnJ on this one... there is just zero point to telling him he's late (unless your L says you need to have mentioned it). He's not going to change. He may even be doing it on purpose to get a rise out of you. There is just no value to pouring any energy down that hole. So what if he thinks he's getting one over on you? Let him. It doesn't hurt you or mean a thing about who you are or how you'll interact in future Rs-- in fact, the work you've already done in acknowledging why this bothers you will be enormously beneficial to you in any and all relationships far into the future. You're getting this out of it while he's just being smug and stupid and shooting himself in the foot by demonstrating with his actions his poor parenting.
You aren't a snail without a shell. You are the f-ing shell and your son is inside. You can take whatever $hit he tries to throw at you because you are protecting your child and doing the right thing for him, even if it means not reacting now, or being nicer in the tone of the letter (I might have a slightly different take on this from OwnIt-- while I 100% agree that assuming you have an L you can trust you should go with their recommendations, on this one I would go with your gut. You know your H and if a little sugar here helps, especially since he doesn't have his own attorney to tell him any differently, it could make a big difference. And don't worry about annoying your L as long as you're willing to pay).
All to say I obviously haven't been through any of this myself and I live in another country. It is like me trying to perform first aid because I watched a lot of Grey's Anatomy. Obviously listen to your L and the vets who have been through this... but I just wanted to give you my POV on this. Remember-- you're the shell. A steel-reinforced one at that.
Me (45) H (41) M:13 T:17, D8 & D10 4/19 BD #1 ILYB 8/19 BD #2 Long-distance EA 12/19 BD #3 Actually 2 year PA 2-5/20 R attempt #1 6-7/20 limbo again, back in contact with AP 8/20-present R attempt #2