@SpoonyOP Maybe you should just get it all out in the open.
But first I would speak to your ex husband, tell him that your current husband is threatening to tell your son the truth, and discuss how you both think it would be best to proceed. Your ex husband is likely to have an opinion on this too, and I'm sure he'd rather hear from you that there's a risk of it all coming out now, than have your son call him up out of the blue and say, "Why didn't you tell me you're not my real father?"
It sounds like he is in fact your son's real father, in every sense other than the biological sense.
Perhaps the two of you could meet up with your son together, say you have something important to tell him, and then just say, "There's no easy way to tell you this, and we probably should have told you years ago. But Dad isn't your biological father. We met when you were a baby and he was happy to raise you as his own. Every time we've considered telling you over the years, you've been at an important stage of life which we didn't want to disrupt, but now you're an adult and you deserve to know the truth. Sorry for not telling you sooner."
I probably wouldn't even mention your current husband threatening to let the cat out of the bag. Just tell your son, in what you and your ex husband think is the best way. And then don't tell your current husband that your son already knows. Let him believe he can continue to dangle this over you, but just grey rock him. Because if you tell your son and then you say to your current husband, "I told him, so you can't threaten me anymore", he can still do damage by speaking to your son when he's still reeling from this news and saying something like, "they never would have told you but I thought you deserved to know and I told your mum that if she didn't tell you, I would". Whereas if you don't tell him your son already knows, he'll probably continue to use the threat of telling him for a while yet, and then if he ever does tell your son, hopefully it'll be long enough after you told him that he will just say, "Yes, I've known that for a while."