We have very similar situations (though I am older now). This feels like me writing to my younger self.
My dad was a drug addict until lung cancer scared him straight a couple years back. Identical situations: debt, unpaid bills, weekend benders. He'd get clean and be great, then fall back into whatever he was doing at the time. And to this day he is always "going to the store" and sneaking cigarettes. My sister and I have the same age difference too, I'm oldest.
The first thing you need to know is that it is not your responsibility to help him. You aren't qualified to do that. know that your dad loves you, but he is a sick man; Addiction is an illness, and his drug use is not because of you. At this point his drug use probably isn't a conscious decision, it's uncontrollable. He needs serious help.
My mom once told my dad it was either us or the drinking, his response was "I'll never quit drinking." It took me a long time to mature into knowing that this wasn't him, it was his addiction. I still love him for who he is underneath that, though I rarely get a chance to see that person.
The next thing you need to do is move in with your mom for a while. I remember many late night phone calls to get my mom to pick us up when he was using- I snuck a phone in my room at 11 years old so I didn't have to call her in front of him. Your dad will probably be upset and heartbroken at this, but that is also not your fault. You need to keep yourself and your brother safe.
There is a chance he won't clean up quickly, a history that long isn't wiped out in a few months. My time spent with my dad while he was using did me a lot of psychological harm because I was young like you, that's a very hard thing to experience As a kid/young adult.
Next, therapy. I'm just starting now, and my 20's have been much more difficult because of it. You may think you're ok, and you are very strong to be handling this and taking care of your brother. I felt like a warrior caretaker, but ultimately I was refusing to take care of the most important person: myself. Therapy will help this. Talking to mom is great but she is too close to the situation. Professional help is best.
Be safe my friend. Tell your dad you love him, and once you're in a safe place with your brother you can lay this out and try to get him to seek help. Just remember it is not your responsibility, or your fault.