You must understand that fear of breaking away from a routine pushes some AS people to the brink of a panic attack. He will always possess this need for routine and rote, a need that is difficult to meet in such a crazy unpredictable world. Here are some ways to help him:
1.) Suggest that he make a pocket timetable for each day. He can then carry the timetable round with him and and look at it to see what he needs to do next without having to ask you. Perhaps his Learning Support teacher could help him to create these? Better yet, advise him to acquire a PDA. (If he can't afford one, he should apply for Disability Living Allowance.)
2.) Tell him not to worry about this need for routine and reassure him that you don't mind helping. If he's apologising constantly he must be feeling anxious about upsetting you - and the more anxious he gets, the more he will cling to the routines. They are his safety-blanket. The more relaxed he is, the easier he will find it to cope without having to ask you for constant clarification.
3.) Suggest that he see a counsellor (preferably a cognitive behavioural therapist who has some experience with AS). CBT is a practical way of making sure that a routine doesn't seize control of your life. It also favours a very logical approach, which your AS friend will appreciate. On some occasions your friend's routines will inevitably be broken. This may be a distressing thought for him, so distressing that he won't want to talk about it with the counsellor. Remind him that it's better to suffer some mild distress now as he learns to deal with the possibilities rather than to endure a messy meltdown when a routine change occurs without warning. Something like CBT will give him the skills to cope with that kind of situation.