One of mine was the same, we both work full time and as far as we knew he was at school, he was leaving to go. It wasn't until his teacher phoned to say he hadn'd been in for over a month that we found out. He had just turned 16 the teacher asked why he was staying on at school, I said he either got a job, went to college or stay at school. In the end we let him leave, took a year to get a job, he is now 20 and working for himself and working very hard, we're very proud of him.
Maybe your son has had enough of school, some do. In Scotland kids can do a linked course with college at 15, this works really well for the ones that have had enough of school but need to stay for their age. This may be the way to go. Ask him.
Good luck never give up, just try a different course.