This is just an outsider view of how the text exchange between you and you H about S came off.
To me, it sounded like you were using H wanting more time with S3 as a punishment to your H. Your anger about H's choices and actions came through loud and clear.
Do I understand this? Yes. Do you feel that if H wanted to do what is in the best interest of S that he should have never left home in the first place?
Do you not trust H to care for your son properly while he is with him? Is S3's safety a concern? If any of this is in question, then by all means, do what you have to do.
I'm going to tell you what I think may happen. If you let your H have your S more, after a bit the newness of caring for him for extended periods will wear off. H will get a real taste of how much work it is caring for a young child. How confining it is to have to meet someone else's needs before your own and how you just can't pick up and go whenever you want. I believe you will get more and more excuses from H as to why he can't take S as much.
Getting a mediator involved will set the schedule that has to be followed for when your H gets S. It will also set the schedule for you, too.
I believe H views the mediator as a 'legal move' and it's threatening to him, therefore he's using going after S 50/50 as a threat to you. Let me ask you this? In your country is getting a mediator involved possibly considered as a first step toward D, or is it entirely a separate issue?
Examine where all this is coming from Red. Make sure you are making decisions about S solely in his best interest.
This is not meant to be a 2 x 4. I truly understand where you're coming from. I've been there myself. Nothing like this crap that makes a Mama Bear want to come out swinging.
Just a little different perspective to think about, Red. I truly think you're doing great in handling a difficult situation.
