You'll probably want a kid-back kit, with a second (third) set of cranks. Or at least footpegs or something. I don't think I've seen any tandems that would fit him right now and I can't imagine liking it if it did. Plus he'll grow out of it right away. You might want to stick with a "pony" for, oh, three or four more years. The great thing about the ponies is you can ride your son to school, then (hypothetically) detach it from your bike and leave it there until you return.
The downside of all this is you'll have to learn how to ride a diamond frame, unless you spring for a bent tandem.