Hi Jeremy!
Are you kidding?! I think it will be a great time to go! Yes, it will be warm, but the heat is very manageable, and your son will have a great time enjoying ALL the sights and sounds!
Walt Disney imagined Walt Disney World with the family in mind. There will only be a handful of rides your son will not be able to enjoy at his age.
Here's a quick list:
Magic Kingdom:
Big Thunder Mountain Railroad
Splash Mountain
Space Mountain
Stitch's Great Escape
Tomorrowland Indy Speedway (requires riders to be over 1 year old)
Epcot:
Soarin'
Mission: Space
Test Track
Disney's Hollywood Studios:
Tower of Terror
Rock 'n' Roller Coaster
Star Tours
Disney's Animal Kingdom:
Expedition Everest
Dinosaur
Primeval Whirl
Kali River Rapids
We've traveled with children at many ages under a year old--7 weeks, 3 months, 4 months, 6 months...and have enjoyed each trip. We usually manage morning nap in the stroller, and then head back to the resort for a solid afternoon nap. We also stick as closely as possible to their 'normal' schedule, and believe it or not, traveling with infants is SOOO much easier than traveling with toddlers (and sometimes 4 year olds!)