If he didn't want any kids, he should have taken precautions. A baby is a shared responsibility.
Yes, you should tell him immediately so the both of you can decide on the next course of action.
Why are you afraid of telling him? Are you afraid that he might suggest an abortion?
My thoughts are that the both of you should sit down and talk this out. It is tiring, but extremely rewarding. I would treat the baby as a blessing even if it was unplanned.
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