@LAC247
I'm trying to figure out just exactly what you're angry about.
You knew about this possible child years ago (before you married?). Did you tell him that you would want nothing to do with this child then? Did you hint or threaten that you would end your relationship if this child was his or make it sound as if you would make a relationship with this child 'difficult' for him? Sounds as if he denied the possibility. Could it be that he denied it because he didn't want to lose you? Not that it makes that OK, it doesn't. Just trying to figure out why he denied it flat out rather than get a DNA test right then.
So time passed, this child was completely 'forgotten' by both of you and you've had your lovely little life and family.
Are you angry NOW because he went behind your back for the DNA test and won't give you the result? Or are you angry because this child has come back into your life and 'disrupting' it?
In a way, you've brought some on this on yourself. If I had been you I would have INSISTED that he get the DNA test way back then and put the matter to rest once and for all. And I would have threatened to end the relationship if he didn't. Instead you were more than happy to merrily carry on ignoring the fact that there was this child 'out there' that was possibly your husband's. Sounds to me as if this is a case of chickens coming home to roost. If you'd dealt with it then, you wouldn't be dealing with it now. Your DC would have grown up with this half-sibling as part of their lives rather than having it 'sprung' on them now.
And why are you (apparently) angry with this child for popping up now? It's hardly their fault that your DH and you have pushed their possible existence into a dark closet and pretended they didn't exist. And since this child was conceived before you even met DH, why are you so insistent on not having a relationship with them? Your DH didn't 'betray' you, so why should you be so angry with this child?
PS, the fact that your DH won't show you the result tells you everything you need to know. The child is his. If it weren't, he'd be happy to share the result and it would be 'case closed'.