We have a Flagstaff HW27KS and it is the perfect size for the 5 of us. Since you didn't specify how old your son is, I will tell you that it takes both me and my wife about 30-40 minutes to set up/break down each time. If he's not able to help, then I would probably advise against this type of model for your excursion.
HW27KSNow, if you're wanting the "tent" experience, maybe a hybrid should be something to consider. The benefits are significantly less set-up time, and if bad weather arises, or you just want to pull over in a Wal-Mart, you can just not fold out the bunks and sleep on the dinette.
It looks like alot of your trip will be in the hot/humid states so to make the trip more pleasurable, I would suggest PUGS. These are reflective devices that fit on the exterior of the tent that will reflect the heat and help keep the insiders cooler during the hottest part of the day. It will also protect the canvas from bird droppings, etc.
PUGSHybridYou should also consider Reflectix. You basically cut it to the size of the bunk window openings and put them between the screen and the canvas. Again, keeps alot of the heat out and does add a minescule amount of sound deadning. When you're not using them, just put them under the bunks.
Another option could be a R-Pod
R-PodYou have endless possibilities of which camper to get, but you always have to be thinking about will it be something that you're happy with 5 years down the road. All campers will have their pro's and con's but you just have to decide what you can live with and what would be a deal breaker.
You have a AWESOME plan, and hopefully when my son gets a little older, I can do something similar.