I agree with all the previous posts. While I am sure he will not be over his marriage by then, it is still long enough away that he will be back to being happy for people getting married. He is going to want his step-sister to be happy and just because his marriage wasn't successful doesn't mean he won't want to help you celebrate your's.
I agree that you have been through way too much with trying to get this wedding planned that you should not have to delay it again. I'm sure he'll be fine and ready to talk about the wedding plans in a couple of months, but bringing them up now is not the greatest idea. I'm sure he'll ask how the plans are going once he is ready to talk about it.
Just give him lots of love and be supportive. I'm sure the last thing he would want right now would be for his divorce to impact another couple's marriage.