Calm down; you almost certainly *can* all board together.
The random selection is usually for extra security, so get him ready for that; he may have to have a brief pat-down and be wanded. (And if that happens, do not fight it; arguing will just make the situation worse.) Also, make sure to double-check his bag and his pockets and make sure that he has nothing in them that would slow him down at security. When you get the boarding pass, you will almost certainly find that it will have the letters SSSS printed somewhere on it. On the unusual chance that his name matches one of those on the TSA watch list, it also would not hurt to bring along a copy of his birth certificate to verify his DOB and US citizenship -- you don't HAVE to, but it couldn't hurt, and might help speed things along.
One of his parents will have to take him to the ticket counter to get checked in. Everyone else in the party can go ahead to the gate and stake out your places in line. When you get to the gate with him, go to the gate agent and explain that he was selected for extra security, and that everyone else in the party has an A boarding pass, then ask if it would be all right for him to board with the rest of you. As he is a child, they will almost certainly allow him to do that. (If he was an adult they would have him board with his group and advise you to save him a seat, but since he's a kid it is very doubtful that they will insist on that.)