First off, look on the bright side. This is an easier task than telling your boyfriend of 2 months, that you're 3 months pregnant. Still, I am not naïve to the reality that sharing the fact that you and your new mate have created a human is a big deal.
The best way to tell him is in simple terms. Give him the facts. He may or may not flip the F out. Maybe he will be psyched and loving from the start. Maybe he'll flip out for a week and then say, "Ok, not ideal, but let's figure this out together."
I think what's key is to take some time before this talk to get the strongest personal handle on the sitch.
You need to do some mental/emotional/logistical/spiritual homework, because essentially you will not only be telling him "We're pregnant" but also, "Now that we're pregnant..."
Buy a bucket of your favorite ice cream and think long and hard about your dot dot dots.
Some sample dot dot dots you may want to meditate on:
Are you going to have this baby?
How well do you know/like this guy?
Are you going to have this baby as a couple?
How are your finances?
How's your support network?
Finally, I'd suggest telling your boyfriend sooner than later; this is life changing stuff we're dealing with, and getting him in on the ground level will work best. You don't want to tell your boyfriend of 5 months that you're 5 months pregnant. Or your ex-boyfriend of 15 months, that he has an 8 month old.