You've got to tell him the truth. You were completely within your rights to have an abortion, it's your body and you have control over it - I think this is something you should have discussed with him beforehand, but that's a moot point now - but you can't lie about it. He deserves the truth, and a relationship can't stay healthy under the weight of a lie like that. Tell him what you did, tell him why you did it: as much as you can, don't lie, don't cover up, and don't omit. That's the only way you can go forward and keep your relationship honest.
I hope you both get through this okay, and good luck.