I can't add anything helpful for the here and now solution unfortunately. Am out of touch with education as my DCs are well grown up now, but thought I'd just post a supportive word.
Right now I'd certainly work on putting thoughts of stepmother out of my mind, you really don't need any negativity or judgement when you obviously want to do what's right by your son.
Easier said than done, but keep trying.
Also please do put the worries about being a single parent out of your mind too, because although it obviously brings trials for you, from experience it doesn't mean troublesome teens with two resident parents and every possible facility, don't also cause problems. What I'm saying is that parenting on your own isn't necessarily anything to do with the situation now. You sound as though you're hung up on people thinking his attitude is 100% your responsibility and yet you can see for yourself that it isn't otherwise your daughter would be just the same.
One of my DCs was also a problem with schooling. Always very popular and well liked by peers and staff, but incredibly frustrating because of not working or taking any work seriously. We literally just tried everything and despaired at the lack of support from college which meant leaving without qualifications.
The turning point was having had a few years drifting and doing nothing, just getting into gear off their own back. It was, again, the old you can lead a horse to water scenario. Now they have a responsible job in a fairly senior role.
It must seem like the end of the world and you don't know where to turn, I remember that kind of hopelessness, but don't fear that he won't be ok in the end. The time goes quicker than you think and it may well be that he finds a niche that works for him. He may well be struggling because he has no idea what he wants to do in life, but that can change in an instant, you never know when something will crop up
Just keep doing whatever you can and know that as bad as it feels, it won't always be like this.