It sounds to me that you are doing the best you can and that is the only thing any of us can to. As far as advice other than talking to his teachers directly to see why his classwork is not being done I can't think of anything to add. I kind of know where you are coming from because I have a son who is 18 now and I still struggle to find ways to motivate him. Same thing here, he is a good kid but lacks organization and motivation.
He wants to work and has been looking for a job but in my opinion he should be much more aggressive than he is. He puts the applications in but doesn't do follow ups like I think he should. He's laid back by nature and sometimes that's a good thing as far as getting along with people but definitely not good when competing for jobs.
I had the same problem with him in his school years that you are having. I knew he was smart enough but didn't apply himself. I went through a very hard frustrating time with that to say the least! I did all I could do but he ended up failing. What made it worse is I was in the middle of a divorce and my ex kept blaming me!
Anyway after he failed I went with him to classes to get his GED (I had quit school so I got mine at the same time) Doing this together kept him motivated, actually he aced his GED exam. We both passed and that was great but now I am trying to get him more motivated in his job search!
He does have a big interest in the things I am doing online and has been helping me out and learning marketing at the same time but I told him he still needs to get a job and a steady income right now. I am paying his car insurance and I have now imposed a deadline that I will stop paying. Hopefully that will light a fire under his but. Anyway, I really didn't mean to make this so long. I can tend to talk to much!
Just keep trying different things and hopefully things will come together. One thing I did do last night is we had went to my nieces for Christmas Eve and she had just bought a beautiful house and has really been successful. She has a college degree and has a great job and I did tell him that if he wants to achieve that kind of success that he has to apply himself and make things happen. I let him know he is a smart person and he can do anything he puts his mind to but it does take action and now that he is 18 it's time to get the ball rolling and start building his future.