The only thing I can try to do is show him constant love and a parental figure he can rely on. His dad is not capable of this right now
You're doing the right thing Cherry. Focusing on being a rock for your son is precisely what you need to do.
Don't forget to put some focus on yourself as well, though I know it's got to be tough with everything you've got going on. Hopefully your WH realizes the damage he's causing, but if he doesn't then your S will need to leverage your stability/steadiness in the future.
Keep it up, we're all here to support you!
Me39 M11 : T13 D9 BD 5/31/16 In House S until 6/21/17 Divorced 10/5/18