Kids by previous relation?

For some reason I can't seem to shake this thought! My man has a potential kid from a previous relationship, he's never met the child and a DNA test was never done so we're stuck thinking is it his or not? I think I dwell on it so much because I want my first child to be his first child but would have to accept this kid if it turns out to be his since the child was born before we got together... I even try doing virtual DNA test by comparing pictures but it's so hard I drive myself crazy lol... any advice on how to get over this?