One of the great things about visiting Walt Disney World, is that you can plan out a vacation to fit within a budget, and still experience the magic!
You'll want to first consider what time of year to visit. There are some "off-peak" times, during which you can find the resorts at reasonable rates. These are usually called value season. If you check in during a value season, at a value resort, you're on your way to having a budget friendly vacation!
Value resorts allow you to stay right in the midst of the magic, where you'll enjoy benefits such as free transportation to the parks, and Extra Magic Hours, all while sticking to your budget! You'll find the great quality of service that Walt Disney world is known for provided by the cast members, and you'll also be making it more convenient for your family to enjoy visiting the parks, too!
When you do head over to the theme parks, it's good to keep in mind how much you'll want to tackle each day with your little one in tow. I suggest going at a pace that makes it easy to stick to your son's regular daily routine. Since it will be your first time visiting, that makes it all the more likely that a base ticket will suit your needs well. There is no need to add on Park hopper options since you will likely find plenty of things to do at one park each day that you choose to visit a park. In case you decide to park hop once you are already at Walt Disney World, you may upgrade your ticket at any guest services window and do so. Now you're set to venture into the Walt Disney world theme parks! I suggest starting off with Magic Kingdom, but if you visit for enough days, go ahead and explore as many of the other parks (Epcot, Animal Kingdom, Hollywood Studios) that you can, as they all feature something that the whole family, even your littlest one, can enjoy!