a) If boy in question, or his offspring, ever does DNA testing on their own, for ancestral curiosity, they may discover this, on their own. Entirely possible the boy too will notice the resemblance.
b) Even bringing it up is rattling your wife. Clearly she expects ruin to come from even discussing DNA testing was done. I'm surprised you discussed it with her, because the innuendo alone can be damaging to a relationship. It says clearly that you suspect she was still sleeping with this guy while married.
c) Continuing down this path with questions and/or testing can have dramatic shifts in all three of your relationships. The sole question here is are you prepared for that. It's already changed your relationship with your wife, and likely will change your relationship with your son. And at best will confirm your 18 year old is or is not your son, and not who is his father if not you. Because if not you, old boyfriend would also have to consent to test.
I thought about letting the son know that his mom once dated this guy. It would not take long before he asked the obvious question, and the answer could be “It’s not that important. I’ve loved you all these years, and always will. If it bothers you, you and I can sometime do a DNA test to know for sure. “
You and your wife have lived, laughed, cooked, slept and vacationed together for a quarter century. That is something not easily put aside. Could you tell yourself to quit worrying about the paternity of the child you have called your son all these years. What would happen if you told your wife it is not important to you who is the father. The only thing you ask is that she tell her ex to quit hanging around - it makes you uncomfortable.
What repercussions did she list out? Ones to you? Or to your son?