I haven't had time to read through all the posts so excuse me if I am repeating some info.
I think this would depend upon your son's interests. If he loves movies and shows then skip EPCOT. However, MGM does not offer a ton to do for little kids, but there are some great attractions. So if he can sit for a good amount of time he can see Indiana Jones, meet the Power Rangers, see the parade, stunt shows, ride on a few rides like TOT, great movie ride, Backstage, Star Wars, do the Honey I Shrunk the Kids play area, Playhouse Disney, Fantasia, etc...
If this is not his thing, then you can hit just the front part of EPCOT as there are a ton of rides and activities for kids. My kids love this part. There is so much for them to do and you can easily spend a good part of your day doing it all. There are shows, 3D and interactive, rides and experiments that would keep any kid interested. Also if he really likes the rides than bear to your left when heading towards the countries and head over to Mexico and Norway as they are the only two countries with rides.
If he enjoys trying different and new foods, then head over to the countries and have him do a pastry/dessert taste test for some of the countries. When we go we hit all the countries, stop at the Kidcot stations, do a project and try a sweet treat for only 3 countries. For the countries we don't eat at, we take a treat souvie for home. We keep a log of which countries we've eaten at for lunch and dinner and try to do other ones on next trips.
We prefer EPCOT, but TOT is one of our favorite rides so we usually do a 1/2 day at MGM and spend the other 1/2 at MK. Our son also loves Power Rangers so it is another reason to go, but they do not like sitting for all the shows so again it is a 1/2 day event for us. I like the thrill rides and Rockin Roller Coaster is great. I'm the only one who can ride it, so DH usually watches the parade with the kids and I fast pass it twice (I use his fast pass as well). It is the only time I get to do something alone
