I have a houseful of boys too, so I understand how that make-up of a house can be BUSY, and how one bad attitude can act as a springboard for another... and another... and another... and if we are not careful, that is the way we behave with one another within this home of ours I so desire to be happy and peaceful. I think it is important to check those behaviors at once, and I really do put my foot down about "bullying" which can happen right within our own homes if we are not careful. Guarding the little ones, so that the older dc are not overly bossy with them, and teaching them to speak kindly with patience and love to littler ones is very important to do, and takes real training, often on a daily basis.
It seems that dc who are closer in age can actually have more conflict than dc that are quite far apart in age, at least this has been the case in our home. It helps me to step back and be calm about it. To talk individually to whomever the person is needing some guidance, and then to calmly and clearly explain what is not acceptable, what change I expect in detail, and what consequences will be given if it happens again.
We as parents have much more control than we sometimes think we do. We can decide what privileges our dc are privy to, and what things they are not. Snacks and desserts for example are privileges that may be taken away. Bedtimes may be moved earlier. Computer time, videos, phones, etc. are all things that fall within our control and are privileges that may be taken away. Activities are privileges that may be taken away if dc cannot behave in a way so deserving of these as privileges. Toys / money / playtime / free time are all privileges as well. Dc are not simply entitled to privileges.
The other side of this coin is that our dc need to feel loved and valued. I have to say that HOD is helping me to take time to truly hold on to their hearts. One thing that has helped is to do each of the boy's HOD Bible time discussions totally privately, without the other boys listening in. Much ground is covered in these times together, and I feel the sharing parts of it where I admit struggles I've had and character habits I've had to work through and are still working through gives a nonjudgmental way for my ds to share his struggles and character habits to work on. I am a get-it-done person, but when it comes to these talking times that are steeped in the Word, I make a concentrated effort to take my own sweet time with it, to not rush it, and to make it count as much as I can.
So, I guess I'd say that first it would be helpful to explain in detail, clearly to your ds in a sit-down type meeting with him alone (along with dh if that is possible), what behavior is not acceptable and what changes you expect. I would make it a key point that speaking unkindly to younger brother will not be tolerated and will result in "x" consequence every single time. Younger ds is a valued member of the family and as such will be honored and loved no matter what. Next, taking account of all privileges ds has that are under your control and deciding which ones to take away would be good. For example, perhaps for every time he says something unkind / mean to younger brother, I'd move his bedtime back 30 minutes. If he is going to bed directly after supper some nights, then he is. If he had an activity to go to that night, then he would miss it. This is the time for clear consequences with consistent follow-ups. No matter what, follow through is key, or he will not take this seriously, and his behavior will not change. Last, I would make sure you are getting time alone with older ds. I would make sure his "love tank" is filled up. How does he most feel loved? When you talk to him? When you spend time doing something special with him? When you write him a loving note or tell him what makes you proud of him? When you surprise him with a small gift? When you make his favorite dessert? Every child has their own way they really do feel loved, and if you can figure that out, you can save a lot of time, and get to doing whatever helps him feel most loved. "The Five Love Languages of Children" is an excellent book that can help you figure this out - it sure has helped me.
Finally, I loved this past thread about this same type of topic, especially "lharris's" response was so helpful. I thought it may be helpful to you too...
viewtopic.php?f=6&t=8437&p=61519
I want you to know you are not alone. I think HOD can really help us in this struggle to hold on to our dc's hearts. I also think prayer is a huge help. I hope something here has helped, but keep on struggling through this to hang on to your ds's heart - it is worth the battle, and the Lord truly is on your side here.
In Christ,
Julie
My oldest was in public school and did the same thing. I'm sure you will get many ideas from others. I will tell you that, the longer this behavior persists, the harder it may be to change. My best advice is to pray, seek God, and decide with your husband a plan of action. Long term consistency is most important. It's tough to see your child being angry and mean. He knows it's wrong or he wouldn't shut off. Set up consequences for when he acts this way and stick to them. Sometimes not listening has natural consequences, and as long as they aren't dangerous, he should experience some of them. If he doesn't listen to directions for schoolwork, redoing it a few times will encourage him to pay attention. However, if he isn't listening to you when you are trying to talk to him about his behavior and God, then the consequences should be decided on. Make sure he knows he is loved, but that that behavior isn't acceptable. I am a children's ministry director too. I spend a lot of time planning and praying to reach other children's hearts for Christ. It's hard when we see our own children struggling. Keep praying. God is faithful. 
I'll pray for ya too.