The one part here that matters to me, given the history, is if you and he discussed his courting someone else before he did it?
If not, I don't really see how what he's doing now isn't cheating. He's just not being secretive about it. Cheating is about violating the boundaries of the relationship and if it wasn't clear that you were okay with that, it's not a very caring move on his part.
If you did make it clear that you were okay, but now that it's happening you're feeling differently, my only suggestion, as simple to say and hard to do as it is, is to talk to him about it. When new people are brought into your world, it naturally causes feelings. It's important to share those with him. Perhaps he's not aware of how much Christmas means to you, perhaps he is.
And more broadly, you've got to honor yourself before you can honor relationships with other people. He has a history of cheating, and even after working out the issues, he did it again. It seems that you also feel that him loving other people means less love for you. If this really is true in your interactions with him then you need to do what's best for you. You said he deserves to be happy, but so do you.