I'm terminally scatty. Here's some of what I do
day to day. Most of my family has similar systems so for us it's normal. ( I was a bit shocked on my early bird plus course recently to discover most people don't live like this lol!)
List on the front door with what you need to take each day. A cheap tesco value white board or laminated sheet is ideal as you can tick the items off before you leave the house. Also pin a "letters for school" wallet next to it so trip letters etc actually make it back to school.
A laminated list that's permanently attached to school bag that states what has to come home each day (this must include the homework diary btw for secondary!)
organisedmum.com does wicked calenders and diaries - they aren't cheap but the stickers are so cool and the layouts are very visual.
Blackboard paint one side of the bedroom door to use for list makers for random reminders for "stuff"
Coloured post it notes are your friends. I post them round my screen and then remove them as tasks are done.
Blackboard in the kitchen to write up when we use stuff up - that becomes part of the week's shopping list.
I used to have a laminated list pinned inside my swimming bag to remind me to shower then dry then cream (excema) as a teen 
It becomes a very effective way of life - I used to earn good money planning multi-million pound projects for a living. If you know any adult/teen dyslexics they'll prob have all sorts of good visual personal organisation ideas.
It is important to teach some strategies if you don't want your adult DS stranded in a strange town with no wallet, or train ticket home someday, it's not just about school. Lots and lots of patience while he learns the systems that help him on your part are needed. My friends used to get incredibly frustrated with me when we were teens due to my scattyness before I'd sussed what worked for me & I'm NT. ASD will make that need for patience greater & you will probably find the teacher expects him to progress at a far quicker rate than he is able. Stick with it though as it will pay massive dividends in adult life regardless of the demands of school. His TA should be helping too with this. If he doesn't crack it till year 10 - so what? So long as he gets there in the end.