Representative Payee Application - received first check before approval letter

My adult son was recently approved for SSI. I went through the representative payee application over the phone with a rep from Social Security. They sent me a copy of the application in the mail. A couple days later I received his back pay check. The check is payable to my name for his name. I have not yet received the letter stating that I was approved as his representative payee and my bank won't let me open an account without that letter. Should I expect to receive the approval letter soon or should I contact the local SS office to see what is going on?

Also is there documentation online from Social Security for how quickly the back pay needs to be spent? The back pay check is greater than the $2,000 maximum asset limit.