I personally wouldn't do 4 parks in 3 days. That said, if you're the types that find running around from early moring to late at night for several days on end to be relaxing (and many people do!

) then it is most definatly doable, as long as you've got a plan!
Here is what I would do. One day each at MK and Epcot, then do a morning at AK then hop to MGM. Both AK and MGM can be seen in less than a day, so you should be able to do just about everything you want at both parks.
As for your day off arrival, since you'll be running around a ton the next three days, and may be tired from traveling earlier that day, you may want to dine at your own resort (AKL, in this case). You can check in, unwind, head to dinner, then spend the rest of the evening exploring the resort or even go for a swim! Then you can just turn in early so you can get up 1st thing the next morning to head for the parks!
But, you'll have to do even more planing than this. If you are going to do sit-down meals in any of the parks, you'll need to plan which park you'll do what day, so you can make the correct ADR's. Also, the parks could still be crowded since not all the schools are back, and even if it isn't too crowded, it's going to be God aweful hot!

The surface of the sun is only slightly cooler than Orlando in August. This makes standing in line torture if the lines are outside (and many are). To combat this, you should get some of the park touring plans, either from the Unofficial Guide (my personal choice) or from Tour Guid Mike (a sponser of these boards). These plans tell you what rides to do in which order so you spend the least amount of time waiting in line and doing the maximum number of rides in a single day. I can't speak for the TGM plans since I've never used them, but I've used the Unoffical Guides plans, they are several to choose from and you can also make customized plans at their website (for a fee) if their published ones won't work for you.
If you don't want to structure your days quite that much, then at the very least I recomend that you, your DH and your son list the rides/show you want to do/see the most at each park. Put them in order of priority, and do all of those rides/shows first, then if you have time left, spend the rest of the day doing other rides/shows not on your list, or repeating your favorites. Or, if you're done with your list for the day, go back to the hotel room for a nap and/or swim, then go back to the park after dinner to catch the evening firworks/parades/shows if you're interested in seeing those (and I sugest you make the effort, NO ONE does fireworks like WDW!).
Either option will make sure you get the most out of the parks. The last thing you want to do is show up at the parks, and stop every 5 minutes to look at the map and ask each other "what do you want to do next?" "I don't know, what do YOU want to do next?" You'll waist a lot of time, and may not be able to do all the things you want to do and THAT would be a shame!
Now, all this said, if you get there and try to follow whatever plan you've made, and you discover that you're not having any fun, do not hessitate to throw the plan in the nearest trash can (don't litter!

) and go do something that IS fun. It's better to waste time in the parks and have FUN, than do the maximum amout of attractions possible and all be unhappy. What's the point in that? Also, don't rush to much running from ride to ride, slow down a little bit and appreciate your surrondings, Disney's attention to detail is legendary, and it would be a shame to miss it while runing to the next ride.
Another tip, if you like to shop ("window" or actual!) save the stores in the parks until the park closes, or your ready to leave (whichever happens first). The stores, especially near the exits, stay open after park closing and you can take the time to look around w/out taking time from park touring.
Well, that's all I can think of off the top of my head. If you want more advice please feel free to PM me! Good luck, and have a great time!
