Work hours too much

My 20 year old son has just been diagnosed autistic.  He started a new job in Sept which was 16 hours which suited him.  he didn't tell them about his diagnosis and wanted to try and "get by".  His hours have now increased to 32 per week and he can't cope.  The problem is he really likes the job and the people but can't bring himself to speak to them and explain.  He would rather leave the job - he really needs to work it's done him the world of good up to now.  Any suggestions how I can get him to speak to his Manager (he won't let me do it!)