First, any woman who would say this to a child's mother, whether it's true or not, is a flaming ******. And if she's a flaming ******, she's probably a liar on top of it. So ignore all of that. Your husband is an immature jerk, but that doesn't mean he's praying for your son to die.
Like the others have said, you need to be smart right now. Talk to a lawyer and save any voice mails, emails, or texts where he says something stupid.
It's possible he wants custody because he doesn't want to pay child support. So you do need a lawyer and you need it asap.