I assume fidelity is important to you in your current relationship?
If so, yes you should tell him - because all couples who have entered into a monogamous relationship should have the fidelity conversation sooner, rather than later. I disagree that fidelity is ever a noble choice, or justifiable, but can see why it happens and why it is a choice for people who feel unhappy in their relationship. I can also see why people overlook the more obvious choice, which is to exit a relationship with their dignity and self-respect intact.
You need to forgive yourself for this, but part of that process I suspect, is about knowing as you do, that you would never make that choice again. Your current partner, if he has any sense, would feel more secure, not less, if you reframe this as a bad choice that you would never make again. People who have learned bitter lessons about infidelity are actually more infidelity-proofed, not less.
In fact why not regard this instead as an enormous gift to your partner? From what you've said about him, he is not a man who would ever use this against you in an argument, or regard your admission as a passport to his own infidelity.
To err is human, but it is madness not to learn from that mistake. I have the utmost respect for people like you, who regret making the choice that you did and have learned from it. I have no respect for people who continue to defend infidelity as a behaviour choice, because that implies no learning and paves the way for them to do it again, with all the hurt and carnage it causes.