Announcing pregnancy to kid who isn't biologically yours?

talisa • 26. Business owner 💎. Married👰🏾. Mommy to one beautiful 5.5 y/o little girl 💕 #2 due 6/2016 👶🏾
I know it's super early, but we plan to announce to my daughter and nephew (we have custody of) before her bday on 11/15, I'll be close to 12 weeks by then. 
I plan on getting them books to prep... My issue, all the books I find are specifically for BIG SISTERS and BROTHERS. I want him to feel included as well, but we don't refer to him as our son.. Any book suggestions? He's also 8, will be 9 on the 18th of this month.