Sarah, I'm sorry, I'm not much help to you, as I haven't been through it before. But I have heard that kids can react this way (just generally being difficult in order to get more attention in reaction to having a new baby in the house). The only thing I can suggest (and I know it is easy for me to say, because I'm not actually going through it!) is to continue to encourage Zander to do the things he can do, but maybe have an activity that you can do just with him (maybe while Juliette is asleep or something) and use the time to make a BIG DEAL about how you are just doing something the two of you. And then also make a BIG DEAL about how grown up he is, and how helpful he is to you...
These are obviously pretty straight-forward ideas that you are no doubt doing anyway, so please forgive me if I'm telling you how to suck eggs. All the best with it, Sarah!