curlygirl,
ATG treatment for aplastic anemia is the tried and true method, routinely successful and usually well tolerated. There are no guarantees with this disease but youngsters are definitely in the most successful treatment group.
You might want to read
Evan MacNeil's profile. Evan was 9 when he started having symptoms and he and his mom were gracious enough to share their stories with us. You can exchange messages with his mom if you want to (see the links at the bottom of the page).
You mention that your son was excited when everyone else was excited. That brings to mind something to keep in mind: that kids take their cues from the adults around them and especially from their family members. If you are optimistic and confident he's likely to be the same and if you let yourself get discouraged he's bound to sense it. Nobody can feel positive all of the time, especially when facing medical unknowns, but the more you can keep your own spirits up the more everyone else will feel that "team spirit". And it really helps!