Business checking isn't necessary (the banks now charge a monthly fee for a small business checking). You can pretty much set up a regular checking account

Depends upon how old he is, you may legally have to be over it for him to sell there. Pay is direct deposited into the account you choose(normally every monday), or through paypal untamedrose.etsy.com

Etsy has a ton of helpful information on setting up shop. I'm not sure if under 18 can have a shop but probably need parents to set it up with credit card and bank account. You get paid in your etsy seller account if a buyer uses "direct" checkout, otherwise through PayPal using their credit card. You have access to PayPal funds and can transfer that to your linked bank account. Funds paid using checkout through etsy is sent to your bank account every Monday. Best bet is read through the forums on etsy and ask lots of questions. Also, read the help etsy offers or email support if needed.