If I had to pay, I'd take him economy, no seat. I know, It'll be hellish, but its only a day. We did it that way with DD 19 months, and me 28 weeks preggers. It was awful, but we lived, and for me flying is always awful, so its a waste of money to try and jazz it up ifyswim.
But with airmiles...hmmm, you know, I'd prob go with business. He probably wont sit on his own seat anyway - when we flew with no booked seat, we actually had a seat for the 13 hour leg, because there were spare ones, but DD sat on DH's or my lap the whole time, and then slept a bit in the floor in front of us.