My experience, when I've been there with kids, is that I save the Magic Kingdom for last and do Epcot first.
Even though they're trying to reverse this, Epcot was designed to be educational, and in the early years it was pretty boring all around. They have Soarin' and the Frozen ride now, which helps. If your son is interested in other cultures he'll dig the World Showcase, but if he's not it'll be a yawnfest, except for the street performers at the various pavilions.
Magic Kingdom was specifically designed to thrill kids, and they love it. The problem with going there first is that they then expect that from the remaining three. The other parks have many more rides that younger kids can't experience, and things they won't get, like some of the streetmosphere performers. LEaving MK to last usually results in a good finish.
All that being said, you do have to look at things like expected crowds, if, when and where you have booked dining, etc. Since you're staying offsite, it's best to avoid parks with Extra Magic Hours, because they'll have more guests with non-hopper tickets throughout the day, making them more packed.