Opinions please?!

My hubby and I are TTC our second child after a miscarriage in December. We've decided that if I get pregnant, we aren't going to tell anyone until I'm between 3 and 4 months along. Our little boy is going to be 2 in August, and he says if I get pregnant before then, he wants to announce it to everyone at our little boys birthday party. I'm fine with this seeing as he is still young enough that it won't "take away from his day" but I'm not sure how we could announce it. (I know I'm getting ahead of myself, I just can't sleep and started thinking about it.) I thought about buying our son some books about a new baby and about being a big brother and having them to open them as a gift in front of everyone and seeing if anyone catches on to it. Any other ideas for ways to announce it at his birthday party (if it even happens) ?!