Should I be with a man who already has a child? What does the Bible say about men leaving the mother of their child to be with the women he loves? Is it a sin for me to be with a man who has a child?
You are asking the wrong questions. The proper questions are: Is he a married man? If he was married, then the question is why did his marriage end?
If he wasn't married, are you talking about being with him as marrying him? Or are you just interested in having sex with this man?
The problem is that you are leaving marriage completely out of your question. "Marriage is honorable among all, and the bed undefiled; but fornicators and adulterers God will judge" (Hebrews 13:4). Without marriage being in this equation, all answers are that it is wrong.
Whether a person has a child or not does not alter the answers, but you should consider what it tells you about a man's character before marrying him.