Secondhandrose said: "Make sure he knows he has to go legally and you will be arrested if he doesn't so he needs to make a choice.". This isn't quite true.
Firstly legally he doesn't have to do anything. The only legal obligations are on your (mumofayoung's) part. Maybe this doesn't matter very much but I think it's important to recognise this and only make statements that are actually true.
And second: deregistration is available on demand. So if you do get arrested (or, more realistically, summonsed to court), it will be as much because of your choice not to deregister as his (possibly correct, possibly not) choice not to go to school.
What is so good about school anyway? Maybe he should go to school, but you haven't made any argument why.
"They have recognised and suggested that he has an attatchment disorder and are arranging an assessment with an educational phycologist." - he doesn't want to go to school therefore there must be something wrong with him? I recommend you read this article "Who wouldn't be school phobic". It applies equally as much to any medical label for children who don't want to go to school.
www.takingchildrenseriously.com/who_wouldnt_be_school_phobic
Also, "he seems to change his reasons why he doesn't want to go". If you put yourself in a position of judging someone's reasons as acceptable or unacceptable, they are likely to change their stated reasons in an effort to find one you find acceptable.
Finally, it doesn't follow that a person has to go to lessons at one school in order to start to go to a completely different school. Why would they?