Of course you love your son and he will always be yours. But you owe it to him to confirm his biological parentage for his future health care. Normally a doctor will ask you about your parents' health in determining your risk for certain diseases.
You need to do this. For your son's protection.
This is a ridiculous reason to DNA his child.
So what if he's at "greater risk" for getting a particular disease. Is he supposed to do things differently if there's a 1/100 chance of getting Huntingtons disease or Tay Sach's disease because it's a genetic disease with a small but not insignificant inheritance rate?
What about all the adopted kids where they know nothing about the parents?
You think even if they DNA the kid and find out he isn't the father he'll have any luck in a) determining who the real father is and b) even if he does the father will cooperate in any way and give him the family history of diseases?
Please. This is just nonsense saying to DNA the kid for his own "protection".
As far as the Op goes- it's not your "fault" she had an affair but yes you are partially responsible for the problems in the marriage that led up to it, if you ignored the signs that she was being neglected and her needs weren't being met.
As far as how you're handling it now, looking for answers you'll never get, and even if you did you wouldn't know if they were true, you're handling the situation extremely poorly. Once you find yourself asking your cheating wife if she ever loved the guy who was banging her, you have already lost.
She may have cheated because she simply has no respect for you, and it appears she might have good reason. Your actions to date simply reek of desperation, and it's just pathetic and sad. Don't expect a positive response from her until and unless you man up. Even then, it's questionable but at least you might have a chance to stay with this remorseless cheater who blames you for everything.