We had a reward system for following instructions.
DS memorized the steps for following instructions:
1. look at the person who gave the instruction
2. say "OK"
3. do the task
4. check back ("hey mom, I brushed my teeth")
Correctly followed instructions (all four steps) get praise and sometimes a little reward. This obviously requires parental follow-through every time.
We started giving simple instructions, then moved to more complicated ones. It worked.
More generally, I hope you have some help (through school or privately) to work on executive function throughout his day-- it sounds like he needs help staying tuned in.
Best,
DeeDee