Hello! I'm a fellow 4-H member (Last year!
I'm too old..), but sorry to say, I have shown ducks for most of my 4-H years. Last year was my first time with a chicken project. I used hatchery quality chickens, because that was the only area I knew to get chicks from, and breeders were often too far away. Aside from that, my parents didn't particularly care to pay for 'fancy-fancy' chickens, let alone drive too far to get some.
For your son, I suggest you use some of your pure-bred Orpingtons for Breed shows, but you can probably use other birds for Showmanship
For Showmanship shows, the judge looks at how the 4-Her handles the bird and how much knowledge he/she has about that bird (age, gender), that breed (history, uses) and chickens in general. I never had amazing SQ birds, so showmanship was my favorite.
For this, use the calmest, most easily-handled bird. It doesn't have to be a specific breed- I've known members who have placed highly with well-behaved mutts and EEs
Breed shows are based solely upon the bird, and how close they are to the standard of perfection (SOP). My chickens were from a hatchery, but they placed 2nd overall. My county's poultry program is made up mostly of hatchery stock, so birds almost never meet the SOP. Judges here are pretty lenient in their judging, because they know not everyone has access or can afford SQ birds, but I've heard other counties are much more competitive, and some amazing quality birds are found at large county or state fairs.
I love 4-H, and its a great experience for any kid. The first couple years are all about learning how to succeed, not about winning. For most of my years, I never placed in any show, but it's still great to talk to judges- they are always willing to give advice for care and breeding.
I hope your son has as great of a time as I've had, and I'm glad you support his desire to join!