That school is for kids that are troubled or who don't do well in a traditional school. I think they work at their own pace there rather than the strict schedule of a regular school. Is it possible he was kicked out of school for the marijuana, maybe his school has a zero tolerance for that and that is the reason he is being sent there. I would be worried that if he is in a school where all of the kids are there because they have been in trouble, things might get worse for him and he might end up getting his education from the other kids. It seems like schools don't want to deal (or maybe don't have time) with kids that are less than perfect students any more, they just ship them off to set an example for anyone else who is thinking of stepping out of line.
Last year my daughter got suspended for 2 weeks and had to go to the Mason Learning Centre (public board) while on suspension. Some of her school work was sent there once she completed it she would just go home. Because not all of her teachers sent work she fell behind in a lot of her classes and had to depend on her friends to help her catch up with notes etc. It may be different for students who are there for the whole year.
I think I would really try to find out the exact reason they are sending him there and see what can be done to get him back into a regular school as soon as possible. If it's because of his grades maybe he could get some one on one help to bring his grades up. Maybe he has a learning disability and gets overwhelmed and gives up. One of my daughters has a learning disability that took years to diagnose, her teachers thought she just wasn't trying. Her 5th grade teacher had a background in special education and recognized the problem and got her into the necessary programs to help her. She did much better after that.
I hope you are able to find him help. It's sad to see kids tossed to the curb because nobody has time to get to the root of the problem.