So sorry about your necklace.
I agree with the other mamas that not hiding your anger, while of course not directing it at him, is the best natural consequence. Let him know that you are sad about the necklace and angry that he didn't listen to you. If necessary, explain that because he didn't do what you said, you are upset now. Pick up the beads with him and ask him to help you repair it.
Repair it with stronger string and a better latch, though.

I also agree that intent here is kind of fuzzy. At that age, he might very well have intended to pull it indefinitely (as long as it got a rise out of you) but not intended to break it. He didn't want to really hurt you, he just wanted to get a reaction.
Very tough. Hugs to you and your son.
