You should tell the truth as you knew it when he was a father to your child. Perhaps you could be convinced that he has matured since then and could potentially be a wonderful father, but you should really tell it how it was in truth, without any negative emotions from your feelings to him as a partner.
The fate and interests of the child is the key, not your ex. The question is, would the child be better off in care or with this man. Deep down I think you know the truth, and we don�t have that liberty.
It seems some responsibility will inevitably be passed to you in any event, would you feel differently whether the child was neglected after you recommended he be placed with the father, or if your ex hated you because the best path for the child was in care.
It always has to be about the child, and the decision will be for the courts, not you.