If the games were bought on your oldest sons account digitally they will only work on his home console
You can sign him in on your youngest console and change it to his home console and then the games will work on it and your oldest can log into his console and still play them as well
Sorry if this makes no sense I’m one of those dyslexics and don’t always write things right
or at least that’s how it worked on PS4 and still works with the new xbox