If he wishes to help that women support that child then he with most likely be traveling back and forth between you and her. If you would rather have him forget her then say that too him but be calm and mature and listen to why or why not he agrees with you.
get a dna test. a few moments of ectasy can turn into a life long consequence. why is he having sex with someone else other than you? what happened to faithfulness? you want to be with someone who fools around? no sugar coating here. dont make someone a priority if he thinks of you as secondary. its a shame people trash relationships. if he is the dad, then support the child. its not the kids fault he was made. take responsibility and support the child.