This is a lose/lose situation:
You lose if you confront him.
You lose if you tell your folks that you know that he has been getting high for some time.
You lose if you allow it to continue and it gets worse.
Your decision is what you are willing to lose and for how long.
Don't expect the Disney after school special outcome. This is real life and rosy, bullshit propaganda won't help you.
What generally happens when kids this age are fingered for using weed/drugs, they are sent to the most expensive drug treatment that your families insurance can afford and it will not work. The only time people quit doing drugs is when they've had enough consequences and young teens have the worst 2 year clean stats of any other demographic.
OK- here's what you need to do: you've got to catch him cold- busted with the goods.
This will give you and your family the ammunition that you need to move forward.
Next- rehab: DO NOT, repeat DO NOT go for a faith based rehab. They are bullshit, long term facilities which charge insurance and then warehouse kids out in the boon docks far away from civilization with no certified staff, just Jesus freaks telling them to pray away their problems. These rehabs are scams and we are working on getting their federal funding pulled. There have been suicides and molestation in them- as they are run by churches or church fronts, they are exempt from a lot of the documentation and certification requirements that other health care facilities are required to produce. Their staff does not even go though a state background check.
The best, most successful rehabs are based on 12-step programs. Others can not boast the numbers or the long term success but they try. Don't fall for it.
There are no easy answers. Steer clear of people that try to tell you that there are. This is going to be a very long battle so dig in, study/learn and expect it to be unpleasant.
When things are coming out, don't be surprised if your own dirty laundry is dragged out for inspection.