I'm having trouble understanding that the parents are Orthodox, but have waited nine years to baptize their child. Really?
This and the issue that the poor boy may have never set foot in church before gives me cause for concern. Is the priest involved or has he delegated it?
Since you appear to be dealing with a lot of unknowns, I would say, since you've asked, that "what" he knows is nowhere near as important as "who" he knows. Does he know Jesus--not about him as an abstraction, but has he been to church? Has he ever prayed to God? What does he himself know?
Once you find that out, you can go from there.
Knowing dogma is secondary. Knowledge of dogma is a support for faith and love, which are fostered through going to church, prayer, him seeing what others are doing and asking questions.
And, what do the parents know? Who will his godfather be? What does he know? How willing are the parents to have his godfather involved?
When I was nine years old, I knew a couple prayers, but more than that I knew the general flow and feeling of the services. I knew a lot of Bible stories. I loved God and church and singing hymns. But all this happened organically. I learned from home, from Sunday school, and from attending church with my grandmother and her friends.