I have to state that I am not a psychologist or doctor, but in my unprofessional opinion this sounds like what is called an "executive function" problem. It's not an issue of intelligence or cognition, it's difficulty in completing tasks, being able to organize one's time or thoughts, being able to plan ahead. And one executive function is the ability to tell a story or give directions without missing important sections of the instructions or the story. That skill requires one to have a beginning, a middle and an end, and to progress smoothly through all three parts.
My niece has Executive Function Disorder, and she too has great difficulty talking about her school day. She'll jump right from "so we were in math class" to "do you think our group should have gotten in trouble for that?". But she gets therapy, and has a 504 plan which helps her plan her work more thoroughly and other assistive aids, and her skills have improved significantly. And oh, she's in 9th grade. And she started therapy and got her 504 plan at about 5th or 6th grade.
You might do some research on Executive Functions and see if this might describe your son. Don't freak out! It's a teachable skill, and actually kids who need a little help with executive functions often benefit when they are older. The occupational or speech therapy they get helps them in high school and college and in a career when all those functions (organization, planning ahead, delivering cohesive and coherent instructions to a team or colleague, managing time, comprehending and following through on instructions, self-discipline and restraint) are invaluable.
Often, Executive Function Disorder doesn't show up in the very early years, because toddlers and kindergarten kids don't have to do a lot of planning. They're learning to sit still, to share, to listen to the teacher, to read and print and use a keyboard, but they don't have to plan projects or give a detailed talk to the class or give an oral report.
It's good that the teacher is aware, and it's good that you are too. It's great that you're willing to see that your child is having a problem, and that you're willing to seek help. That's one sign of a good parent! Your son is lucky to have you as his mom, on his side!