I flew on a BA
777 last summer, and I highly recommend choosing outside seats in a row at the rear of a section or just for of an exit, so whoever is sitting in the window seat will not have to step over someone else to get out.
Looking at Seatguru, it looks like rows 62 and 64 for the upper deck and row 20 (lower, but it's over the wing) on the 52J configuration and 62 & 64 (UD) along with 14 and 20 for the 70J configuration. Row 62 may not work since it is an exit, but I don't know exactly what seats are considered to be in the exit row with this config.
As far as easiest conversation goes, seats in the middle section of the lower deck would work well, but I'd bet that most 9 year olds would prefer a window seat. For the outer sections, I suggest getting a window (rear facing) and aisle (fore) in one row, and the aisle of the row behind. This should allow relatively easy conversation with both of you, assuming your son is in the window seat, the downside being that he would have to step over whomever (you from what I gather) is sitting in the aft row aisle seat.
Here is the Seatguru.