Hi Ann,
The average temperatures in Orlando in March are:- High 79° - Low 56° so all being well, it should be nice and warm without being too hot during your visit.
There are lots of attractions which your 5 year old Son would enjoy at all four of the Disney Theme Parks and so I would advise that you take a look at what is on offer at each
Theme Park to see which three your Son will find the most fun.
I would definitely recommend that you visit the Magic Kingdom as that really is the heart of Walt Disney World. My now 9 year-old Son has also always loved Disney's Hollywood Studios especially Toy Story Mania which I have no doubt that your Son would like too.
When it comes down to your third Theme Park, I would say that if your Son is particularly interested in animals to visit Disney's Animal Kingdom Theme Park and if not, visit Epcot Theme Park instead.
Wishing you a magical trip filled with memories which will last a lifetime!