Ouch, you definitely have a situation here.
1) He's almost 16 so he's probably itching to be independent and just think adults don't know anything in general and he knows everything...you know, like most teens. :)
2) I wonder if it's because of the 2 year old and the baby to be. Maybe he is questioning where he "belongs"...something else I think teens need to have a clear sense of.
3) As for the schooling, I would remind him that if he dislikes school so much, he can keep failing because when his friends are moving on to the next grade next year and working their way towards getting OUT of school, he will still be there repeating Freshman year/Sophmore year/etc. until he passes what he needs to to move on. My 3rd grader began to dislike school a bit this year and thought we were just "picking on him" about his writing and various things. Once I told him if he doesn't score enough on his various classes he may have to repeat 3rd grade, he greatly changed his attitude because he did NOT like that thought.
4) Because of the grades and pot, he should definitely have his fun things restricted (computer, phone, friend time, etc.). He can earn them back as his behavior and grades improve.
Have you tried talking to him yourself. I understand you work a lot but is there anything that can be done with your schedule for a while until you help him get under control. If he dislikes his dad and step mom so much and only likes his stepdad when they are having fun, it sounds like he could really use some serious one on one time with you and a heart to heart. Remind him constantly of how much you love him, tell him every time he does something right - helps his 2 year old sibling, helps clean up after dinner, does something without mouthing off...no matter what it is, try to find things to praise him for. Studies have shown that praising children tends to make them make better choices more often because they want to keep getting that praise. Also, ask him what he wants for his life. If he wants to be a doctor, he can't have failing grades and colleges look at high school transcripts. Does he want to be "one of the stoner kids" who ends up not doing anything with their lives or does he want the world to be open to him in any way he wants?
The hard part is you staying calm to talk to him. Trust me, after the morning I just had with my one son, I am reminded how hard that can be. When he makes the wrong choice we try to use the "what kind of boy do you want to be" and usually it helps but this morning he was seriously testing me and I couldn't hold it together. I got very upset with him. So try to talk to him and it may be like a block of ice. If you feel like you aren't getting through and/or you find yourself getting worked up, walk away. Just chip away at his attitude a little at a time if that's all you can do.
I wish you the best. You know, I was so upset that my son and I had the tiff we did this morning and at one point he said I didn't care about him because I got tired of asking him to do things to get the day going. So because I stopped asking him to do anything and began to walk away from him, I suddenly didn't care. While they waited for the bus though, I pulled him aside and I told him, if I didn't love him I wouldn't get upset. I wouldn't be hurt when he makes the wrong choices. The day I stop caring about his choices, that's when he needs to question my love. I then reminded him that I love him so much that I would lay my life down to protect him. I honestly think that helped him a bit because you don't make that type of claim to just anybody and he knows that.
Again, I wish you the best. I am expecting my 4th boy and we aren't that far from the teen years with our oldest so I definitely feel for you.
I almost forgot!!! Has he been tested by a doctor for being bipolar or anything like that? The reason I ask is, a friend has three children and the oldest has been a genious since he was born. I'm not kidding you. He was a well behaved, brilliant boy for as long as I can remember. We moved 6 years ago so I haven't seen him in a while but we still talk to the parents. Once this boy went to college, everything changed. He began using drugs, questioning authority, failing school (really???)...come to find out he is bipolar or schitzophrenic they believe. The one therapist told them that with people as brilliant as their son, that can happen because they can't related to society. He thinks rules are stupid, society ideals are stupid, etc. I'm not trying to scare you but it's one more thing to consider looking into.