Hi. Just been catching up with events and delurking to voice a possibly irrelevant and irrational unease... it's just the thing about him making out that he's the main carer (not sure if that's the proper term used)...
From HIS standpoint re the letter to you, rather than seeing it as him having to compromise, could he be setting up a situation where he's "granting you the opportunity" to step up your parenting/highlight your lack of time for ds2? Could he argue that he's chosen to put ds2 first, before gainful employment, whereas you, to all intents and purposes, are more interested in putting work first and childcare second?
I have no experience of this kind of situation at all, but I just get this feeling reading his random comments that he wholeheartedly believes he is the "superior" parent in all this, that if he wasn't doing what he does then ds2 would be floundering in your care. My fear would be that he could, in his warped way, enable those in authority to see and understand his point of view.
Maybe I'm overreacting but I wouldn't give him an inch, wouldn't underestimate him and anything that he might have up his sleeve. I just feel he's on a bit of a softly, softly mission, and is playing a long game...
Maybe I'm missing the mark here but it seems to me that this is the time for you to kind of detach and step right back from yr ex's approach/terms of reference. Use this opportunity to get your own clarity, to hone and focus on the exact terms you want for ds2's ongoing wellbeing.
Imagine this time next year.
What will life be like? How do you see ds2? How do you see yourself? What can you do now to ensure you get that best possible outcome? Look at how far you've come in recent months. Now is the time for you to sow seeds, for you to invest in your future with ds2, and, yes... with yr ex doing his bit too...
Apologies if this is a bit ranty, but I was a bit spooked and i'm a bit tired but had to comment...