Keep in mind that your son already knows half of his family medical history - yours, and there's a 50-50 chance he knows the other half, your husband's.
If you know your husband's blood type, and yours, the other guy's (you could get in touch and ask him), and your son's blood type, you might be able to conclude that your husband definitely isn't the father, or the other guy definitely isn't the father. It depends on how the blood types sort out. If you all have the same blood type, then it won't work. You can look it up, or confidentially ask a doctor.
If you want to be sneaky, you could probably get a medical opinion about whether your husband is the boy's father. You'd need a family doctor you can trust, you'd need some reason for your husband and your son to get blood drawn for medical labs.
Ask a doctor or two how important it really is for your son to know the rest of his family medical history. I think most doctors will tell you it isn't actually very important, except in very rare cases.
When my wife and I had kids, we had "genetic counseling." The health insurance company offered it for free. Nice of them... It was a complete waste of time. There were no genetic concerns. It seemed the main purpose was to reassure anxious young parents. This is the usual story with genetic counseling.
You could also ask the other guy if his family has a a history of heritable diseases or rare and dangerous medical problems. If he does, you could make some kind of a written record, make it available to your son if he ever has a medical problem or after your husband's death, or whatever.
I have lived for a long time, and I've known a lot of people. I've never known anyone who got a life-saving diagnosis or medical procedure because his family medical history was investigated. It happens, but I doubt that it happens very often.
Ideally, your son would know his family medical history. That's better for his long term health prospects. On the other hand, he's thirteen. If your husband learns about this, there's a pretty big risk of marital problems, possibly leading to divorce. You know your husband better than we do, so you have to make your own assessment. Assuming your confession puts the marriage at risk, family turmoil is also bad for your son's health. You'e got to find a reasonable balance between those forces.
If you have any Christian faith (not very common on Reddit), Jesus will forgive you for your indiscretion, fourteen years ago. So would Buddha, and so would most normal Americans. I think you can forgive yourself, too. But those are the easy questions. The more important question is whether your husband would forgive you, whether or not he is the father of the child. If he is not the father, he will likely have a hard time with it. He will probably continue to love your son, but some husbands would not forgive you. It's a hard truth.
Personally, I don't think a confession for its own sake, or to relieve your own guilt, or for the sake of your son's health is a good enough reason to risk your marriage.