My son's mother left me almost 6 years ago and I did not know where she was. I tried everything to get in contact with her or her family and had no luck. I had no contact with her or my son for almost 5 years. Since she left and did not let me know where she went or was will I have to pay back child support?

If you haven't heard anything it must be that she never applied--
---or they don't know where to find you?

I am sure if she applied then you would know.
If she does ever apply I don't know how far back they can go.
You might want to set up an account to set aside money for him if you ever do hear anything or if you ever do get to meet him someday.

It is possible for her to latter sue for support and ask the court for back payment, so you need to keep records of your searches for her, and attempts to find your son.

Also you may wish to consult an attorney as to filing for divorce and other legal issues that may protect you