He strongly believes it was not his child.. How can I believe this?

We have been married for 10 years and my husband cheated on me. All seem well until I started suspecting about him.

It was my strong instinct about him having another woman secretly apart from me. Until one time he was overseas I ran through his computer and saw this woman and child.

I had chills going through my body and thought wow this could be it. When I inquired with him and he did not know what to say.

He finally told me it was a one night stand and he did not mean what he did. I continued to asked why he has the picture of the woman and child and he said he wanted to prove it is not his child.

He strongly believes it was not his child. How can I believe this?