Since you know this is a problem right now, YOU will need to be the one to not wait last minute to do anything getting your son ready.
I used to make deals with our daughter. I will do one shoe if you will do the other. Lets see who can run to the car the fastest. (this is from our front door to the car parked right there in our own drive way, not at the grocery store parking lot). How many hops to get to the car? You can button your shirt when we get you in your car seat.
And then sometimes as the other moms said, you just have to say, "This time, I will have to do this".. If he throws a fit, put him in the car and let him scream. Take earplugs. Once you get to where you are going, then have him put on his own shoes.
Ask him for patience. "Please be patient with me, this time, I need to comb your hair, but I promise next time you can comb your hair."
Try to avoid the "we only have 10 minutes to get dressed".. Instead you go to him an hour before and say, "ok, we need to get you dressed right now.."
IF he is ready right away, so what? If not and it takes 45 minutes, you still have that 15 minute cushion.
My husband has ADHD and always will, they never grow out of it.. I am in NO way saying this is something your son has, but it is exactly how my 50 year old husband behaves..
Not with the tantrums, but if I tell him we have to be somewhere in 30 minutes.. all of a sudden he will begin some new project, try to finish what he is doing but realize he needs to fix the loose floorboard he just walked on, or oooo, , "Dr. Who" is on the tv and this is the one part he missed last time so he is going to get dressed while watching the show.. and I will walk in and he is sitting down watching it.
He cannot find his shoes.. the shoe lace is broken.. OMG!!!.. it is like my worst nightmare.. I am like you, I am prepared, I have planned, I have studied the situation in advance. I have my list on my ipad and on a note I printed out.
And so after all of these years I have learned to tell him, we are supposed to be there in 30 minutes.. when in reality it is an hour or more.
Most toddlers are like this. They want to do this themselves and are insulted that we do not allow them to do it their way and on their own.. They do not have a concept of time.. We are the ones that rule the time, so you can decide how long it usually takes him..
Also model the behavior of asking for help from him. "johnny, I need your help, please put my keys in my purse. Thank you! Wow, I am glad you helped me.
Please go to the kitchen and bring me my cell phone. Thank you! You are a good helper.
"Please carry these wash cloths to the bathroom, I will carry the folded bath towels." "Thank you, boy that helped me a lot, you are a good helper".
Hang in there. You will figure out what works best for your son.. Just figure out what he responds to.. With our daughter. noticing her good behaviors encourage good behaviors. Telling her we would take turns, worked for her, Sharing worked with her. She liked being acknowledged and encouraged.
When she had melt downs or stubbornness.. I acknowledged her feelings.
"I can tell you are frustrated that I would not let you walk while we grocery shopped today. Next time we go to target, you can walk holding the basket or my hand. "
"I can tell I hurt your feelings when I had to put your shoes on, but we were in a hurry and did not have extra time." When we get home, you can wear any shoes you want outside to play in. "
Only make deals you are willing to keep.. and if you tend to forget, tell your child, "Remind me when we get home, that I told you, you could wear your cowboy boots the rest of the day. "
"Remind me the next time we are Target, I told you you could walk holding the basket or holding my hand. " This puts it on your child to get and then take the responsibility of remembering.
Our daughter has an excellent memory.. so this worked for her.
He will see that even grown ups need help. That is is good to ask for help and to also thank people that help us.