Alcoholism Support Group
Alcoholism is the continued consumption of alcoholic beverages, even when it is negatively affecting your health, work, relationships and life. If you think alcohol is causing you to lose control, it's time to seek help. Our group is a safe place to vent, check in, get back up if you fall, and reach sobriety.

If he truly feels unsafe or is experiencing emotional or physical abuse when he's with his mother, I would take those concerns seriously. Since he's only 15, this may be something that needs guidance from a trusted adult, school counselor, attorney, or your local child protective services, depending on the circumstances.
I also encourage you to look for a local Al-Anon meeting. Al-Anon is for family members and friends of people struggling with alcoholism, and you'll find others there who have faced difficult situations like this. They can offer practical support as you decide what to do next.
In the meantime, keep letting your brother-in-law know that he's loved, heard, and that none of this is his fault. Having even one safe, caring home can make an enormous difference in a young person's life.
Wishing all of you strength and wisdom as you take the next step....Hugs