Is your young adult learning to drive?
When you have an SPD/ASD young adult who lacks/finds it challenging to concentrate, try the following tips and see if it helps:-
Before going out in the car do lots of heavy work (calming)
* Pushing/pulling exercises
* Deep pressure (massage etc)
* Brush the path or hoover
* Blowing/sucking
* Drink a thick milkshake through a straw
Proprioception (muscle memory)
By adding weights to feet/wrists this would help the person with their spatial awareness to identify where their body is in space (make sure the weights are huge though!).
Some people even chew gum which keeps them alert!