Homeschooling is going pretty well....

Except for trying to keep a certain 7yo boy motivated and occupied. I am running out of ways (creative ways) to keep him interested in learning and what we're doing.
Now, logically, I know that while he is 7, his mental age is at about 3-4, so that accounts for some of it. I've begged, pleaded, bribed, and done pretty much everything I can think of to try and get him doing something.
I've took tv and made it only educational programs, I've taken video games and made them mostly educational. I've made flash cards with easy words, letters and numbers. We've done "field trips" to the zoo, the aquarium, the museum, and other places. He shows no interest at all in letters, words, numbers or any of that. He does know some numbers and letters, and knows what the flash card that says "DS" means (go figure!). I'm at a loss as to how to do things to help him learn.
The other kids did stuff about presidents and the elections, the Whitehouse, and how the government is the one that makes the laws and such (not too in detail, but the basics). That whole time, he was totally letting everyone know that he wasn't having any part of it. I've tried having the other kids doing something and (them) asking him if he can help. I've tried making him sit and holding him in the chair so he would sit long enough to let the cd rom that we were doing catch his attention.
Nothing seems to work.
The other kids are doing well. We are working out of some books that tell what words they should know by when, but alot of our work comes from things that they decided they wanted to know! If something catches their attention on tv, we write it down and it becomes part of tomorrows lesson. They love watching shows like "How It's Made" "Cash Cab" "How Stuff Works" "MythBusters" and all those type of shows. Oh, and "Dirty Jobs"! Thats a favorite of even Gabe! Gabe also likes Sesame Street, Dragon Tales, Wonder Pets, Dora, Diego, Backyardigans, Little Bill, Barney, and Teletubbies! And also Thomas, almost forgot about him, we only recently started watching that one, I can totally see how some kids can't deal without him, lol.
Of course since we don't have cable (yet), all they can watch is what's on PBS unless they're at grandma's house. Any ideas?????