Just to re-iterate a point made by a previous poster... don't be so sure that someone will move from a window/aisle to a middle. If you asked me to move out of my window seat (especially if this was an overnight flight) I'd make it clear that I was happy to keep an eye on your kid, to make sure he got his drink and meal, fetch mom/dad if there was any trouble, even get him to the bathroom if necessary. Basically, I'd be a willing, free baby sitter. But you'd be hard pressed to convince me to move to a middle seat for 10+ hours.

I think you need to decide how mature your son is. Many 7 year-olds would be fine in a middle seat a few rows from mom and dad. A classic family story for us involves how (30 years ago) my 4YO brother sat several seats away from me on a PanAm 747 from Tokyo to LAX. My parents were elsewhere in the plane. My brother was fine except that he talked non-stop at his middle-aged seatmate for hours on end. I felt so bad for that guy.

If you feel that your son is not going to be OK without you or your spouse next to him, then you've just got to pony up the $$ for two E+ seats next to one another. It is the only sure bet you've got.