I have seen things that you can put on the wall, they look like a book type of thing. They show months and numbers and can remind you of things that happen. You can even right things on them with something called a pencil because it can write stuff when things are not horizontally flat.
Does he have a phone or calendar? You need to really check and understand what people are doing in your passport and whether it is the expected stamp or date. Check it straight away, so that it doesn't need to be fixed later.
You put every date that immigration make in your passport into your calendar.
My 90 days goes in my phone and 2 weeks before my phone says do you 90 days online. The next day it also says do your 90 days online.
When I get approved back from immigration I put the next date 2 weeks before the expiry date.
Life is simple if you set up your own procedures and follow them.