You need to sit down and have a talk with him-and don't make it a scary talk like "you're acting weird". Just sit down and listen to him. When I was in middle school I was basically mute too.
He's probably having trouble with making friends or something. At that age friends bascially define how important you are-at least, that's what all the kids think at that age.
Tell him that he's just like everybody else. That everyone is shy. Maybe you remember a shy moment in your life. Tell him about it and he woun't feel so alone about it.
Make sure to always tell him that you love him and that he's smart ('cause dang that kid is smart) every day. Try to make him laugh by telling him jokes. Tell him to losen up and talk about all the people you know who had it worse than him and did amazing things anyway.
Buy him some cool clothes he would be proud to wear in public and at school. He'll look forward to school then. This will make a giant difference.
As many times as you can invite his friends over to you house so they can hang out. He will be closer to his friends and he will probably be a happier, more sociable person.
I hope I answered the question. I'll pray for your kid because I know how difficult he's got it. (=