Joystick for any flight sim and esp Elite Dangerous.
Might want to look at getting a HOTAS as well. I recommend the Thrustmaster T.16000M FCS Hotas & Joystick as a good setup and perfect for ED and may I suggest that if you are interested in flight sims, then IL2 BOM is worth a look. https://il2sturmovik.com/ The flight modelling is particularly good
And don’t worry about your son needing to learn base motor skills, Kids adapt so quickly that it doesn’t matter what controller they are put in front of, they will get the hang of it very quickly.