I was a school refuser from y9 onwards.
It got worse in y10 and y11. My attendance in y11 was 39%.
For me, personally, whilst there was an element of depression, part of it was the realisation that I could learn the same thing in ten minutes as I could in an hour's lesson - I remember being sat in a geography lesson making an information poster and thinking wtf am I doing here - I'm fourteen years old and basically sat here colouring in.
I think I may have reached the conclusion that 75% of school was a glorified babysitting service (I don't think that now, but did at the time) and was, therefore, a complete waste of my time.
I did have problems at home, including a total breakdown of relationship with my mother, but that was a bit of a chicken and egg situation - Ie the non-attendance contributed to the breakdown at least in part, whilst the problems at home exasperated the lack of attendance.
It probably didn't help that despite poor attendance, I did well in exams - didn't acheive my full potential, particularly in maths and science but still high achieving. This only confirmed my viewpoint.
This was in the early 2000's btw - I don't think there was any threat of my parents getting into trouble for it, to my knowledge at the time.
What would have got me back into school? My only thought for my situation was that at college my attendance was much better. This was due I think to a rapport I had with one of my tutors. It sounds cheesy, but he really inspired me and most of all I wanted to 'prove' myself to him. As at this point I had left home at sixteen and had social services involvement, he was assigned as my mentor for additional support and it really helped.
Are there any particular teachers that your son really likes/respects? Could the school involve him or her in mentoring your son in a pastoral care role?
Ultimately I think you need to know the reasons your son doesn't want to go to school - easier said than done. If someone asked me at the time, I think I would have just shrugged and said 'don't wanna. It's shit'.
Good luck 