Port Orleans is a beautiful resort, enjoy! You are correct, that they will not have Diet 7-Up, WDW is strictly Coke products. They will have the diet version of what every the Coke equivalent is, and you can get ice water at any of the counter service food locations. You should not have a problem getting your PD supplies delivered, just make the arrangements with your supplier. You might want to give the resort a call and let them know you are coming. You will need to have your supplier mark your name on the boxes as well as your arrival date and reservation number. To get the Guest Assistance Card (GAC), stop at Guest Relations at any of the parks, or City Hall at Magic Kingdom and explain your son's situation to them. They will give you a GAC for him that will provide for his needs. There have been some modifications to how they handle special needs guests, you might drop by the DISabilities forum over on the DIS Boards (
www.disboards.com) and take a look through some of the postings to get an idea of how things are being handled. They keep their information pretty up to date. There are even a couple of threads having to do with PD on there that might be interesting. I post over there as DVC DOC, so maybe I'll see you there too!
Most of all, take your time, relax and enjoy! It's a vacation, not a death march