S.B.
Hi! 20 minutes is definately not enough time! I start my boys about 45 min to an hour before we leave. I have them get up and take a quick shower (this helps wake them up) while they shower I get some breakfast out for them and after they get out they eat and then get dressed, do their hair, brush teeth etc. Then we are ready to go! I also have school clothes, lunches/snacks and backpacks ready the night before, this helps to not tie up the mornings and they go pretty smooth.
No matter what you do he needs more time and a strict schedule every morning :-) My oldest is ADHD so I understand! Good Luck