Sometimes, severe social anxiety can make a child actually not want to speak. I would have him tested to see what's making him do this, so that you can help him. Good luck.
My son is 4. They are complicated! Keep in mind he can still understand YOU even though he chooses not to communicate back. Whether or not you decide to get him tested, keep talking to him! Tell him what you're thinking about, spend some time just with him in neutral territory,--at the library, getting ice cream, taking him to a movie etc. Give him options for signing instead of talking---if it's an attention getting thing he can at least avoid the screaming fits that way until he grows out of it. From what I hear, middle children often go through phases of desperate attempts at communication.
It also never hurts to get them checked out by a professional as long as they aren't made to feel different or freakish. Make sure his older sibling does not know what is going on if you take him in to get tested. At 7 they tease, even with the best of intentions and that may drive him further inward. I hope my 2 cents helps.