Emptybee
your feelings are completely valid. he was dishonest with you from the start. had he told you from the beginning, just as you said, you could have factored that into your choice as to whether this was a relationship you were willing to take on. springing an additional unknown child into the family is a stressor on any relationship, this one intensified by the fact that your partner was dishonest with you initially. complicating matters more is the fact that he is putting this seemingly adult (?) child before you and minimizing your feelings about the situation.

it seems that you have two options. you find a way to accept the child or make the choice of leaving yourself. difficult choices.

good luck.