If she's the local bike what does that make him? He chose to have sex with her while he was living with you and while you were pregnant with his child.
You say this happened when 'there was no chance for any thing' between the two of you, yet he found a chance to make something (namely, a child) happen between him and another woman.
On a practical level if there is genuine reason to believe that the child may not be your dp's. he is best advised to organise a paternity test.
If the child is his, I hope you will find it in yourself to welcome him/her into your family and try to build a civilsed relationship with the child's mother.
When you found out what he'd been up to you were understandably extremely hurt and said many things in the heat of the moment but this doesn't make you a bad person.
I expect you've had his nuts in the wringer over his affair; personally I'd give the handle a few more turns before I was satisfied that he'd at least think twice before he drops his pants for another woman.
Once my trust in someone has been destroyed I can never being myself to trust them again, but only you know whether you are prepared to forgive your dp for cheating on you, and only time will tell whether you can trust him again.
Unfortunately, no-one can undo the past and you are going have to find a way of living with the memory of his duplicity, and the consequences of his stupidity, if you intend to continue living with him.