Honestly there really isn't a nice way to tell him. That's why it's just easier to tell him straight out that you have a baby and it's also his child. In other words, just get it over with and be honest.
If he reacts badly, then calmly ask him why's reacting so badly. If it's because he doesn't want a kid, then you have to be strong and confront him that he is also responsible for the child's well being and he needs to man up about it. If he doesn't want to, then you can actually take some sort of legal action against him. If it's because he's just surprised, then let him cool down for a second and leave him alone. News like this would be hard for him to take in at first and it's best to not disturb him while he's trying to settle things out. However he reacts, try to remain calm and try not to get angry or anything. Because the more you freak out and get emotional, the more he's going to freak out too. It would be hard to do this, but if you want to prevent him from reacting a lot worse, then you have to control your emotions.
I hope this helps a bit. I've never been in your situation so I don't know exactly what to do, but logically this is what I think you should do. Good luck!