I am an avid tent camper, though I've only camped at Fort Wilderness once. Here are my thoughts on your questions.
1) Well, it depends on your tolerance for the elements. It's Central Florida, so the weather, bugs, etc will be the mildest in spring and fall. Should be somewhat less crowded during those seasons too. Summer will be crowded, and also boiling hot, and winter can be quite cold. Of course, a fan or a heater in your tent will keep you comfortable whatever the weather conditions. If you go during summer though, plan for tons of rain, and don't leave anything outside the tent while you're gone that might get soaked.
2) I stayed at Fort Wilderness for a week. I've camped in a tent for over a month though, outside of New Orleans in the middle of winter.
3) I don't have kids, but my cousins tent camped pretty much since birth. It's never too early, IMHO, especially if your son is pretty open to new experiences.
General thoughts on tent camping and camping at Fort Wilderness... setting up is really easy, even for one person, as long as you have a newer tent. Definitely practice first, but it'll be second nature in no time. I assume you have camping experience? Make sure you check and double check your camping packing list, which should be completely separate from your regular vacation packing list.
One specific note about Fort Wilderness. They DO NOT allow campfires in any way, shape, or form. We tried to build a small, contained fire in the charcoal grill, and a most un-Disney like CM came running over with a hose, informing us that we had 30 seconds to put out the fire, or he would be hosing down our entire site. Of course, we retaliated by putting out the low-burning wood fire, then dumping an entire bag of charcoal in the itty-bitty grill. We doused it in lighter fluid, and boom! A spectacular fire that almost grazed the treetops

Boy was that CM mad, but we weren't "technically" doing anything wrong, so he couldn't say a word.
Other than that, FW is kind of like a deluxe KOA. It's definitely not the back-to-nature experience, but it is a kick. There are lots of activities, there is a group campfire now and then, and of course since it's Disney everything is sparkling clean, the campsites are very level and pretty with manicured grass, it's a nice place. You and your son will have tons of fun!
Any other questions, feel free to ask!