What he said!
When your boy sees that 500mm GUN he's going to go nuts. Warn him you can't shoot one of those things hand held unless you have like 1000asa film. He'll need to lay it down on a bean bag or you will have to get him a STURDY tripod.
I know if I had got my hands on a big lens like that early it would have serriously rocket boosted my interest in photography. Serious nature shot machine! Gets the girl's notice as well. Works like a porche.
His photog teacher may not have much use for it though. They like to start kids with 'normal' perspective lenses which for the 35mm format is 50mm. Its called normal cause it sees about like the human eye does. Little or no distortion too whereas the greater the variation from normal one goes, either way, the greater the distortion of the image.