Thank for responses - I haven't been able to respond sooner as I had a CAMHS appt for DD and then DS is currently super suspicious that I am posting about him.
lougle sadly at the moment DS isn't coping while he is at school. (Although there is also the issue that some of his anxiety is about not going to be able to cope, which takes up so much energy and causes him so much stress that it affects his ability to cope IYSWIM.) The not coping takes the form of becoming panicky which gives him distressing physical symptons (feels his head is 'exploding', rapid heartbeat etc). He also becomes readily upset - sometimes because of his ASD difficulties in coping with all the social and communication stuff within class and also because of his sensory sensitivities.
I think that at the moment DS is finding school as difficult as he fears it will be. He has lost any ability he had in 'recovering' from a 'difficult' lesson and the stress just continues to build. On Monday he struggled in the first 2 lessons, went to the learning support base and the SENCO tried to calm him. As he was still upset he did work from the 3rd lesson in the library. He went back to lesson 4 but then at lunch he was miserable and stressed and went to a place he has found in the library where he can be upset without being noticed. 
And sadly ATM the moment he has stopped enjoying activities in school he used to like eg drama club and choir. Even in the subjects he likes he usually has some niggling worry.
claw and OneInEight I got to speak to the Head of Year today and he has said that a restricted timetable is an option. But apparently this means DS being in school full time but doing the lessons he finds difficult (most of them currently) in the learning support room, or the HOY's office. I had in mine more of a reduced hours option. I agree that it is best for DS to keep going in albeit for less hours. (Though at the moment DS is becoming so tired and depleted that he is becoming unfit to go in at all. Which is why I am desperate for a 'plan'.)
Today I tried to get DS in school for the last 2 lessons - which at his school means from 12 to 3 with a lesson each side of lunch. We drove there DS getting increasingly agitated. He couldn't/wouldn't walk into school himself as he had said he wanted to. I said I would phone learning support to get someone to meet him at reception. He said if I did he would make a scene and not go with them. He was super stressed by this stage saying if he did go in, it would because I had 'made him',so he wouldn't be coming home again because I was cruel. And he was suggesting that he would run off over the common which I had parked next to, rather than walk into school. I said it was his decision and we could go home if he wanted. I felt mean making him decide, but I thought he had to make the choice. He agonised, had to be prompted loads to come to a decision and eventually he said he couldn't do it and we came home.
The going in late doesn't work at present because DS can't cope with the change in routine - and worries about having to explain where he has been to his classmates. What DS wants is to be allowed to come home if he is upset. Which the HOY says can't happen as once DS is in school they want him to stay. I don't know what I think - I obviously don't want DS to continually ask to come home - ATM he might consider himself upset 'enough' to come home each day! There again, I don't know if he is just looking for a 'safety net' if things get too tough.
Please tell me what sort of support the school should be offering. He has a leave-the-lesson pass which he currently won't use because his classmates will notice him go out. He also says going out for a short time won't deal with his panic. He can go and speak to the SENCO at breaks, but he thinks that she is minimising his difficulties - sitting with a friend in a lesson isn't enough and he doesn't want to be taken to the lesson. He has been allowed to do a couple of lessons in the learning support room - he found that hard as there were unfamiliar children and an assistant there. On my other thread homework suggested a 'learning support mentor' which I think might help if it was someone he saw in private. Apparently there aren't any LSAs in his lessons because he is in the top sets for core subjects. Anyway, he wouldn't accept LSA help/attention in a lesson.
The school have suggested he sees the counsellor - but I'm not sure he will agree to do that or that it will help - CBT at CAMHS for DS last year was not a success and in theory the therapist had experience of ASD. I have wondered about medication - OneInEight - how old was your DS when it was prescribed and what was it? My DS2 with HFA was prescribed Sertraline at 14 and has continued to take it - 5 years on....
BTW I didn't get DD into school either. She is much less eloquent about her difficulties at school than DS, but she said school was 'scary' and we had tears. I decided that the CAMHS appointment was enough for her to deal with today and kept her at home - which unfortunately did mean she witnessed a lot of DS's panic - and was probably making mental notes on how I dealt with it....
DS was very tired this evening. He wants a 'normal day' tomorrow but is agitated that he can't do it. If he doesn't go in first thing I will take him in with me when I go to meet the HOY. I am a bit bemused that the school seems to be making a distinction between SEN support and then the HOY who is in charge of 'pastoral care'. For DS they definitely need to be 'joined-up'.