This is harder than usual.
Would you want to know, @Charly108? I would. I'm always on the side of telling, because I would absolutely want to know. I couldn't forgive someone for hiding it from me.
But, I can usually see the argument for not telling, of bearing your own cross and not causing chaos in someones world if it's definitely over and you know why it happened... but in your case, it happened because you've been together a long time, you're bored and curious, and those are things that are likely to reappear.
In addition, he already suspects. It's a horrid thing suspecting someone you love, feeling that something is wrong and not knowing what. I think to prolong that feeling would be cruel. You'd end up gaslighting him, trying to convince him that the truth isn't the truth.
What are your next steps here? Do you intend to break up with him? If you're leaving him anyway, you have a stronger case for not telling him, although I would. If you intend to stay, can you build a future for this, when it's already failing, and it'll now be built on lies? It's going to be a lot harder to do, and you're going to need to be very careful that you don't end up cheating again - it's much easier the second time, you've already done it before, that moral barrier is gone.