Have you explained to your son why you want to homeschool? Maybe make it his choice in that he can either begin to particpate at school and start working or he will have to be homeschooled and not see his friends everyday. Maybe by explaining both angles fully he will reach the understanding that things cannot continue as they are and he has to pick the lesser of two evils. GCSE grades are asked about at every job interview. It depends what he wants to do in life.
Or maybe you could start there. What job does he want? What is the route to get there (apprenticeships, college, university etc). Then explain how GCSEs support that. Mayve having a goal will help.