Busteed was commissioned a brigadier general on August 7, 1862. He was nominated as a federal judge for the US Middle District of Alabama on January 5, 1864 (he resigned his judgeship under threat of impeachment in 1874). So the photo would have to have been made in the latter part of 1862 or in 1863.
His son, Richard Busteed Jr., was born in September, 1855 and would have been 6, turning 7, at the time that his father received his commission as a brigadier general. Despite the caption of the photo, the boy is too young to be a real drummer boy, even by Civil War standards, and is not wearing a real uniform. Most likely, the drum was a photographer's prop (it was common for photographers in this era to have props for people to pose with, and this was especially true of children). The cap might also be a photographer's prop.
I think there is little doubt that the boy in the photograph is General Busteed's son.