Before you worry about telling him, have you told those closest to you?
I am assuming you aren't together anymore as you said he was your then boyfriend?
Have you told your mom? I guess your sister knows from your previous post.
When I told my baby daddy, I knew it wasn't what he wanted, but also I knew him well enough to plan how to tell him, I wrote him a letter and forbade him to read it until he was somewhere safe and unlikely to be disturbed. I promised in it that I wouldn't try and ring him or anything and just waited until he got home. 7 hours later her turned up. I cried, we cuddled, ended up in bed

and we're still together.
I guess if he needs to know you will just have to tell him outright.
Be prepared for the typical guy re-action though - that he will deny it being his, will say you must have slept with another guy and such like.
But once you have told him, unless he wants to pursue it further leave it.
Just my personal opinion and some will disagree I know.
Weigh up the pro's and con's - what can he offer? Do you want him involved with the baby (if you are indeed pregnant)?
Is it worth a paternity test to get him to pay up with child maintenance payments and so on?