I'm a fan of checklists to systematize mundane things I'd rather not commit to memory. It sounds silly, but you could post a little checklist in locations where he tends to forget a step. Have him quickly review the list when entering or leaving those places. For school, give him little laminated business card sized lists of tasks for the beginning and end of the day to store someplace accessible, like his agenda or wallet.
If he protests, you can tell him that doctors and fighter pilots use checklists to good effect. This is an enjoyable read: The Checklist Manifesto.
If he has good spatial memory, having a dedicated place for everything and keeping it in its place is a godsend.
