I think the core of what he said is true. You have been through so much with him, for so long, and you have been hurt so badly. Right now he needs to complete his treatment (he is just starting) and he needs to make a commitment to change his life… and through that you will see by his actions and behavior just like he says.
I don’t see manipulation in his message, more like just a hope he is holding for the future. But since your trying to step away for now to work on your issues, I would just let it go, or gently wish him well on the work that is ahead of him.
My husband did 3 months inpatient, from our experience family and those relationships were a part of his recovery. My husbands rehab encouraged the interaction, but it gained in momentum as he progressed with his recovery. Most important to realize, it has to be something you want also. If that never happens for you, then he will come to accept it.
Sounds like your son had a wonderful birthday. Hold that memory close, and think of how you want more of those joyous moments in your life.