After 17 years, I may have a son

I got a call that my ex girl friend says that her 17 year old son is mine. We she got pregnant in 1989 I asked for a paternaty test but she refused. To this day she has refused. She just recently got divorced from her husband whom I understand raised the boy. I do not know if it is mine because the man she married and 1 other person are possible fathers. Can I get a paternity test without the mothers consent? What if he is mine, what about back child support? or would their be any? I have a wife a 2 children (13 & 4) now. I not exactly sure what she wants since she hasn't directly talk to me. She has made these statement to aquaintances and friends of mine. We have lived in the same small town all of these years, and never a word. I guess I am a little confused and don't know where to start. Any legal advise would be greatly appreciated!

You normally will not have to pay back support under these circumstances but could be required to start to pay for the future or even for college if any.