Routine will work well for him.
Preparing him for things outside of the routine will help his adapting to new things.
If he has a bad day, it is exceptionally likely something about the day is different. It can be something which you or I wouldn't notice, but it will impact on him significantly, changing his whole perspective of the day, sometimes creating a situation where he will struggle to function.
For example, if you always do the shopping saturday, and then take him to visit a relative on the way home, he will become accustomed to that routine. If you don't shop, or visit the friend on the way home, it'll throw his whole world out of sync. Even if you did things the other way around, it'd have a big impact on him. If you didn't need shopping, find an excuse to take him to the shops, just get a pint of milk or something, stick as close to his expectations as possible. If the friend was not going to be in, tell your son repeatedly/daily for as long as you can in the build up to the day.
Essentially, this allows him to prepare himself, and thus have an element of control.
Most importantly, listen to him, and observe him, and communicate with him. My stepdaughter has a non-verbal autistic lad. He has additional difficulties as well. We communicate with him via makaton. She (SD) isn't what most would call an academic person, but she has developed an almost intuitive way of understanding him, and I cannot imagine how difficult and/or challenging it has been for her. I'll see if I can ask her for tips. But be aware he will be trying to communicate with you all the time, it is just that frequently he will struggle with doing this.
