You were very young and it was relatively early in the relationship when you cheated however 3 months is long enough for anyone, esp young people, to fall in love and for there to be an expectation of faithfulness.
If he has been faithful himself ..... From his point of view (if you tell him) you cheated on him, lied by omission about it, lied to his face when asked for almost 12 years (including while bringing a child I to the world with him) and deprived him of the chance to end the relationship and build a healthier relationship with someone else and have kids with someone else (who he might still be with).
That sounds very critical compared to other posters, I know .. but I'm seeing things from his point of view if you now come clean.
I think he had that (correct) instinct all along and that's why he kept asking/suspecting etc. if men do that to women on this forum it's called gas lighting, cruel etc etc. Potentially affecting someone MH because their instincts keep telling them something is true, but someone who's supposed to love them says it's not.
He has been asking you to be truthful for more than a decade and you haven't. He has even more of an issue with this subject because of his childhood .... I think, I'm sorry but; you are fucked if you tell him.
You will never get back together, he will gate you and not trust you.
If you don't tell him and get back together, he will revert to his suspicions/lack of trust/to that feeling gnawing at him ... You will continue to feel guilty etc etc.
There is no getting back together for you too.
Whether you tell him or not
I'm sorry, but that's my impression of how this will play out.
At this point, you could tell him - with acceptance that it will scupper you ever getting back together - but you will wreck the good will as co parents (at least for quite a while).
(If you'd told him the truth after you cheated, he'd most likely have ended the relationship anyway ... Due to typical reactions and due to his Mum/childhood, so the result would've been the same as now. Only difference is you have a wonderful child and can hopefully manage as decent co-parents.