You'll need to definitely build up your son to sitting on a trail-a-bike for that long. He needs to be able to do an all-day trip without, well, doing what 7 year olds like to do "Daddy, I'm bored now..." Not to mention that you have to figure out how long is realistic to ride between breaks.

You can use a trail-a-bike with a rack and panniers and put a rack and panniers on a trail-a-bike, so you can hopefully get away with out pulling a trailer, too. Instead of camping, you can also consider hotels along the way.

It's not completely crazy, but it's not a slam dunk, either.