Go on the Maury show.
one of my mares had sex with this one stallion two times, then she got with this other one, she had a colt, I can tell from which stud its from, but she can't, you don't sound any different.
I don't think you should say anything! If you tell him, he will want to know if its his kid and if its not..he will leave your cheating butt! Also, even if it is his kid..he still might leave you. Best thing you should do is live with the guilt, guess you should have thought of that BEFORE you screwed him over.
If my g/f ever cheats on me, she better be smart enough never to let me know, otherwise she will be left on the side of the road. Once that trust is gone...you won't ever get it back.
Don't say anything. Just let well enough alone. And don't let assholes like the one above make you feel bad either. Shit happens. Us women are aloud to get horny and think with our pussy's just like they think with their dicks. There is no need to every say anything.