I recommend that you start with elementary aged books even though your child may be gifted. Even if his sight reading, ear training and theory are excellent, there are a lot of things in piano playing that take time - the finger position, hand-eye coordination, posture and just all of it coming together. If you jump in at a higher level or use an accelerated course, all of these things may or may not fall into place for a child. If after you start teaching a course at the beginner elementary level and feel that your DS is getting it all and needs no repetition to advance, you have several options:
1. Condense that level to accelerate to the next level
2. Jump to the Accelerated Piano course of that publisher
3. Use option #1 but add in more repertoire to include various artists and styles of music
From the little experience that I have, Faber's Piano Adventures series and The Music Tree series are pretty good. We use the Faber PA series now after graduating from preK level Alfred, JT and Little Mozarts. I love how Faber has supplementary books like the Developing Artist series, Gold performance books and the "PreTime to BigTime" series. They also have an "Accelerated Piano Adventures" which might be an option you can pursue if you find PA too slow for your child.
I also add in really fun, interest driven music books that boys my son's age like (he is 6) - e.g. Harry Potter, Star Wars, Pirates etc in addition to Mozart, Beethoven etc.
We have a teacher who is happy to teach "extra requests" and flexible enough to fit these into her curriculum.
Good luck.
