K.R.
Get a lawyer
There is NOOOOO way to deal with a divorce like he is trying to build, without a lawyer. It is the rest of your son's life that is at stake, and you soon-to-be-ex clearly hasn't got the reality of mommy-loss in his picture. However much it costs right now, any long-term costs would be greater if you *don't* get a good, will-fight-for-you lawyer.
My now-Ex was writing stuff down about me and I *didn't* know it until later ... hello, *you've* (temporarily but massively) destroyed my emotional stability and then you are writing down when I act emotionally unstable, like it's something inherent to me and a reason to take the kids? Not so much!! Hopefully (and presumably) the courts see this scenario ALL THE TIME.
Get a lawyer.
(I managed to defuse my ex far enough that we settled out of court via a mediator. I wouldn't necessarily recommend it, and I think if I had it to do again, I would have acquired a lawyer earlier in the process instead of only at the end to 'check my work' as it were.)
If you are in Portland, I liked my lawyer, who walked a very fine line between expressing what a jerk my ex was being and accepting that I was not interested in being aggressive (just assertive) in *my* part of the negotiations/settlement, even though he came on (initially) aggressively.
(The divorcing partner is often hurt and angry enough about [whatever] to truly feel their demands and revenge are justified ... if he has any sense or self-objectivity he'll start looking for balancing advice soon ... we can hope. God bless. Especially your little boy.)