I'm 38 weeks pregnant with my second and my first is 8yrs old, so I'll have a similar age gap between my two. My situation isn't quite the same as yours, but my husband is not the biological father of my first and I can at least relate to breaking the news to an older child. So what I did was have a custom t-shirt made for my son announcing that "I'm going to be a big brother!!" He was very excited to get the gift and to be able to wear it to school to show off to his friends.
If you decide to have a custom t-shirt made, I highly recommend Brand X T-shirts on Mill Ave in Tempe. You get to design EVERYTHING and the t-shirt is ready in an hour or so and looks so professional!
As far as explaining to your 9 year old who the dad is, I don't know that you really need to approach this topic unless your son asks directly. Focus on the fact that your family (from your child's perspective) will be growing from the two of you to three people; and talk about how this will be an exciting new adventure that you will be embarking on TOGETHER. If your 9 year old asks questions, answer them honestly and directly, but you don't need to burden your child with more information than they may be prepared for.
Best of luck to you!!