See the GP and ask for a referral to the Children's psychological service. If there is a long wait, and there may be, then, if you can afford it ask for a referral to a private child phsychological service.
This child obviously has mental health problems and the schools he was at should have picked this up and made the referral themselves. I think it is unlikely that your daughter could be summoned or fined because your DGS is not attending school because the reasons he is absent is so obvious due tto, what I would describe as, severe mental health problems.
He could also be suffering from depression, possibly as the result of the breakdown of his parent's marriage and his fathe has a new partner and clearly has lost interest in him.