All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
First of all, we ask Allaah to guide you to the truth and open your heart to Islam.
As regards your question, then Zina (fornication or adultery) that you and that man had committed is a grave major sin, but the child born out of Zina is traced back to his mother and not to the fornicator.
However, the result of the DNA test has no effect as the sperm of Zina is not respected in the sight of the Sharee’ah (Islamic Law) under which Zina does not have any effect, so it establishes neither the lineage nor the obligation of spending on this child born from Zina. However, this does not prevent the fornicator from doing good to the child born out of Zina [by taking care of him and spending on him].
Since this is the case, there is no need for DNA tests as nothing is established by it in the Sharee’ah, such as the lineage or the obligation of spending (on the child) and so forth. So, the child is your child and it is you who is obliged to spend on him and not the fornicator unless he spends on him as an act of kindness for the child. For more benefit, please refer to Fatwa 85430.
As regards the fiancée of that man, it is not permissible to lie to her about him; rather, you should say the truth about him and inform her about his conduct so that she knows the truth about him.
Finally, it is a good occasion here to call you to Islam which protects the woman and her honor from abusers. So, Islam has provided for that a security shield of chastity and good conduct, like the prohibition of her being in seclusion with alien men, the prohibition of her not wearing Hijab (Islamic dress), or her being soft in speech to men and so on, as a way of preventing Zina which leads to such disadvantages that you are suffering from now and from which your child born out of a sinful and abominable relationship would suffer.
For more benefit, please refer to Fataawa (plural of Fatwa) 81979, 88906, 88051 and 85456.
Allaah Knows best.