When people try to get their way by threatening suicide it's not a good idea to change your mind and give them what they want. If you resume the relationship then you reward him for his behaviour and he now knows how to get what he wants. So he makes the threat again and again.
Eventually he'll ask for something which you're really not prepared to give him, and you say no. So to show you he means business, he males a suicide attempt, and initially he survives. So the cycle continues, but each time he has to make more and more convincing suicide attempts to "prove" he means business. Only he knows whether he really intends to end his life with any particular attempt. But each more convincing attempt is more likely to cause permanent injury or death. Imagine if he becomes permanently and severely disabled in a suicide attempt in the future?
Now if you go back to him you might satisfy him this time, but you ultimately won't stop suicidal behaviour unless you agree to always do everything he wants for the rest of your life. In which case you're not really alive, you're just his slave. And even then he may think that because threatening suicide worked with you he's going to try to do it out in the world with other people. You can't stop suicidal behaviour by giving in to him.
So I don't know whether he will actually make an attempt if you don't go back to him. But if he actually makes an attempt, or even if he injures or kills himself, you aren't responsible, and you don't have a duty to resume the relationship. If he gets his way you're just postponing the day when he makes a suicide attempt rather than making it possible to learn that these attempts don't work.
Don't go back to him because of his suicide threat. This suicide threat is an additional reason not to go back to him. Instead tell someone else about this threat, for instance the suicide helpline and give them his number. Or tell the police. Or tell one of his close friends or relatives.