You may want to get some testing done to make sure there isn't a learning disability that is getting in the way. My sister in-laws' son was failing all his classes in middle school despite being an otherwise good kid. He would just stare out the window during class and when trying to do homework. After some testing, they found out he has a sensory processing disorder. He has made great strides after only a few months of occupational therapy. Lots of learning disabilities go undiagnosed well into MS and even HS.
The other thing to watch out for is mental health issues. Puberty is about when things like depression can start showing up. Without bombarding him with poking and prodding tests, a session with a counselor might be in order to see if he needs professional help with something.
Last thing is to see what his school environment is like and whether there are alternatives available in either magnet schools or private schools. We live in an age of "drill and kill" education where teachers are spending all their time in the class room trying to make sure everyone is ready for standardized testing. I have seen kids really open up when they were shifted to a different style of teaching, like Montessori or project based learning. Middle school is really a time when a lot of kids, especially boys, get lost because they do not respond to the classroom environment. People frequently believe that these kids are just being lazy or rebellious when they really just need a different learning environment.
Finally, if weed is legal where you live, you should embrace your son's ambition to open a weed store. Take him to meet the owner of a weed store and a grower. He will quickly find out that running a business, even if it is just selling weed, takes an education. And cultivating commercial grade weed is actually very heavy on science. Your grand bargain may be to let him continue to smoke if he gets his act together in school.