Dude, first off, get off his back with the texting. I'm really telling you, you gotta let him live out his life, whether he's slowly drowning or really living, you gotta let him live the life he's choosing. Telling him how bad something is won't change him. people love to tell others how bad their actions are...you know what? I already know all that. I know cigs can kill me, I know I won't be able to breathe and I'll suffocate and have black flegm and risk a heart attack and all sorts of things. I still did it anyway. I knew all the possibilities of pot, DXM, LSA, and alcohol, and I've done all that too. Telling me the bad that it can cause me doesn't make me want to stop. It can prevent a person...maybe once or twice per one hundred drug users but I assure you...when a person wants to do a drug, those aren't the reasons most people don't do them. People aren't detered from drugs through stuff like that...it has to be an honest to God change in their heart. A real, "I want to do this because I'll be healthy, because my friends love me and don't want me to be hurting like this, because the Creator loves me and wants to heal me."
Get back on all your texting...it costs money first off...which you can save for other things, and also, it gets annoying...it really does. I've learned the hard way that if you want to hang with someone or be with someone, give them a call or max two txts, and then let it go. Bugging them to check on them, or ask about stuff constantly is just insane. Worry about him, pray about him, but cut down the texting, it's for your best interest. It sometimes drives people away when someone pursues them too hard and says a whole lot of stuff they don't want to hear.
I whole heartedly urge you to put the drug issue you aside in all conversations and just be his friend. The more you show that you just want to chill with him and help him out no matter what, the more he'll want to chill with you because you don't annoy him. If you're always talking about drugs or whatnot, stuff he doesn't want to hear about, he won't want to hang with you. The most important thing right now is to make sure he's not totally falling into trouble, just be the strong force who stands by him through thick and thin.
As far as coke goes, I'm still doubting he does it. His mood swings sound more like rapid cycling bipolar or some other disorder...he could feel guilt, pain, depression, anything...so many moods could be flowing in him, so much emotion...he's holding a lot in.
His parents: I don't care HOW MANY TIMES you go to church, unless you show that you got God in you, you're time at church is just a waste of time and space and energy. His parents might claim the label "christian" but I doubt they follow the hard road. I've seen plenty of alcoholic christians, pedofile christians, child beating christians, angry, greedy, envious, hateful, cruel and so many other nasty 'christians' out there that I'd rather hang out with atheists (no wonder Jesus hung out with the 'crap' of society right?). I've had better conversations with my atheist brother in law than the hardcore christian at my school. Personally, I'd rather someone call me an atheist than a christian. These are the kind of people his parents might be. As far as abuse goes, it can be mental, emotional, or physical...it doesn't matter which it is unless bones are being broken, he's getting bloodied up or being killed is a possible issue (then you need to get child protective services involved IMMEDIATELY). They are all pretty harsh tortures for anyone...they turn a human being into a wimpering, angry, pathetic mess of brokenness. They end up falling into drugs, bad relationships, homelessness or poverty, mental disorders, self injury, and/or suicide. It depends on the extent of the abuse and how hard he takes it. He parents may not be trying to abuse him per se, but having an angry mother who is yelling about things all the time is not easy to handle and having parents who yell at each other is probably even harder (this is know from personal experience as a horrific torture to the mind).
Just stand strong bro...it's gonna take time, it's gonna be rough, but you just gotta be strong, rely on the Lord...he's the only one who can get you through this...spend a good deal of time with the Lord, read his word, focus on scripture and prayer and meditate on the word. If you are not strong in the weapons of God, it will be hard to stand by your friend. You have to be well nourished in God's manna and properly armed and defended by God's armor to stand strong against the Evil One.
The Lord is with you, do not fear, do not grow weary, stand righteous and in love.
It also sounds like his mom is always yelling and freaking out in the household about stuff. 
It does show that God's in control.
