I have often wondered whether the boy was an enlisted soldier, or a civilian, maybe Busteed's son, dressed as a drummer.
I don't know if he was a soldier or civilian, but I did measure the photo a bit. My head is about 9 inches (22.8 centimeters) tall. The boy wears a cap, but I estimate that he is about 5.5 to 7.33 times as tall as his head, thus making him about 4 feet 1.5 inches (125.7 centimeters) to 5 feet 5.9 inches (167.3 centimeters) tall.
If the drum is the size of a regulation drum it should be about 16 to 18 inches (40.6 to 45.7 centimeters) in diameter Te boy is about2.75 times as tall making him about 3 feet 8 inches (111.7 centimeters) to 4 feet 1.5 inches (125.7 centimeters) tall.
I don't know how tall General Busteed was, but I assume he was between 5 feet 8 inches and 6 feet 2 inches tall. The boy is about 0.68 as tall, or 3 feet 10 inches (117.4 centimeters) to 4 feet 2 inches (127.8 centimeters).
Thus the three height ranges overlap at about 4 feet 1.5 inches (127.5 centimeters).
And that is a very small drummer boy, but there were some that were only 4 feet (121.9 centimeters) tall and even shorter.
A drummer that short could carry the drum slung behind their shoulders when not drumming, and being hollow the drum would probably be lighter than the backpacks many children that size wear to school these days.
Anyway, I thought that I would contribute to the discussion.
Richard Busteeed was a brigadier general in 1862 to 1863, when his son would have been about 7 to 8. And maybe he sometimes wore his uniform later.