Don't expect, what is developmentally, not his age.
This is too young.
They don't even know, what 10 minutes is. Literally.
They think of something yesterday.. .and it is still current to them.
3 year olds, are like that.
They also do not have, fully developed "impulse control" yet either.
1 hour or 10 minutes to them, is not tangible to them.
Kids need to be assisted at this age. They can't get ready, on time, themselves. So monitor the timing of things, and egg him on, and get ready ahead of time, so that he/you are not late in leaving the house. Do things/get ready, ahead of time. Not down to the wire.
Toddlers, even older kids, dilly dally in getting ready.
My HUSBAND, takes longer to get ready, than ME!
So, with him, he needs to be egged on too.
He has NO concept, of getting ready, ON time, either. And he is an adult.
Amazingly, he is never late for work.
So, keep expectations, age appropriate and per your child.
Young kids and even older kids, getting ready in the morning... can be like pulling teeth. Sure it can be frustrating. But, give it time.
Your son is young.
Kids this age, do not know, "time management."