You need to know why he did it but I think you also need to show it's not acceptable either
He could be the fact he's missing you all and doesn't want you to know so he's acting out, I know if rich is nervous or upset he always gets angry first.
I'd sit him down and have a general chat, or find a book about bullies and see if he says anything, I've always found a book with an issue helps to start the talking, your local library will have a list of them for you to choose from