I have a DH like that, and here is how I handle him:
I would start hinting that I would really want to spend some quality time with him alone. It has been SO LONG since we had datenight, and we are SOOO preoccupied with the kids. I could REALLY do with a little US time. Go dancing, and to dinner...... but alas: there is not much time now, and ofcourse the money can be better spent otherwise......
As soon as the invitation drops in, he will remember that I really want some sort of date/adult time with him, and he will probably figure out that the wedding is just what the dr. called for. If he does not figure that out, I will make sure he hears it from me: "Ow! Wonderfull! This wedding is JUST the thing I have been looking forward to! No kids, dinner, dancing!"
I am not sure if it will still work in your situation, because you already have discussed the wedding.