First let me say that I KNOW how you feel about promises and the like not coming to pass.
This is why I've suspected CJ of being "Passive Aggressive". Seems to me that often he WOULD agree to stuff just to shut me up in the moment, or even to please others (sure I'll copy your 8mm films onto VHS MIL!...two years later....Sure I'll print out digital photos!....three years later....Sure I'll do June's monthly expenses! Two months later the stack of receipts is still there....Hell, Tal he told me a week a go that he would "cut back significantly on his school work and get that done "tomorrow"...Nope.
Now I know this is "little stuff", but it's a similar pattern and it drives me crazy! I'd rather not have him promise to do anything at all than go back on it!
So...I HEAR you!
That said...there probably are some ways you can soften your letter so as to make him more receptive to your POV.
How about starting with some of the GOOD stuff that's been going on? How glad you are for the progress so far (and it HAS been substantial!)
Then perhaps tone down the expectation that this might "cause a big fight"...maybe just say that part of your general mistrust, which MUST be dealt with relates to what you perceive as this tendency of his to not follow through on promises.
Now how about giving him a chance here and asking if he recalled your discussion about when he'd move back in? His head might have been muddled and he might not recall it as clearly as you.
Let him know that YOU took it as an understanding that he WOULD be back in when S starts school...ask how he interpreted that negotiation.
Explain that perhaps what feels like something "promised" to you, is not that concrete to him, and how could you come closer so that each of your needs are met?
The basic message is the same, just how it's coached. Leave him some space to regain dignity here (even if he IS coming up short too often ). Try to write this so that it OPENS communication while expressing how you feel. Quoting Tal:
now I hear you saying things that lead me to believe you are dragging your feet, being vague, and not even recognizing that we did, in fact come to an agreement
See, this I think is almost guaranteed to make him feel accused and lousy. ...So again, ask him what HE thought your "negotiations" meant....