Toilet Anxiety in teenager

My son has not been officially diagnosed, but has all the signs he is somewhere on the spectrum. In recent weeks he seems to be experiencing some issues around toileting. To the point last night he didn't go to sleep till 2am as he is couldn't stop trying to go for a poo.

Basically he will go to the toilet, think it is just a wee but then end up sitting there for over half an hour as he thinks a poo needs to come out and he is scared of getting off the toilet and having the rush back. Now normally this has been manageable and he has been coping, however last night he finished, got in to bed then felt the need to go back and the cycle started all over again. He did this about 4 times (each time sitting on the toilet for around half an hour) until 2am when I finally managed to distract him when he got off the toilet so he didn't think about it and we managed to go to sleep.

This "obsession" with the toilet has got worse over the last 12 months, he has always had a little bit of anxiety mainly around going to the toilet before going to school and being worried he may need a poo when at school. However this has now escalated and seems to be consuming his life every time he goes to the toilet. 

I am just concerned for him, how can I help him overcome this fear, how can we help him get back to a relaxing bedtime. I am also dreading his return to school as his would only be sat on the toilet for around 10 - 15 mins before Christmas, now almost every time he goes he is there for around half an hour. Which will increase his anxiety about being late for school and make him need to sit there for longer scared more will be coming out. It feels like a never ending cycle.

Has anyone else experienced anything like this, is there something I can do to help him?