RE: From how many knots does S-Type 137 planes at 87kg
Unfortunately, the Aero 127 is 74 cm wide (considered very wide for a loose and turny wave/B&J board) so I think it's going to plane a bit earlier than the 72 cm wide S-Type 137 (72 cm wide).
But I don't think it's realistic to plane on either of these boards, at 87 Kg.
in 10-11 knots of wind.
With a good solid 12 knots, and the 8.5 m2 Retro, he can probably pump a couple of times, or flutter pump a little bit more and get on plane with good technique, but less than 12 knots for 87 Kg. is probably the domain of wider boards like the '06 Isonic 135 (80 cm wide) ; the '07 Isonic 133 (80 cm wide) and Isonic 145 (88..5 cm wide).
These boards could be made a little more "loose and turny" with a more maneuver oriented freeride type fin, but they will never be as loose as the Aero 127.
Have you tried some larger freeride type fins and alot of different mast foot positions to get a bit better early planing on the Aero 127.
I've had the Aero 117 and found it a bit "sticky" in terms of planing early, but with careful fin selection and experiementing with mast foot positions I was able to improve the early planing characteristics by around 1 knot windspeed down to 11-12 knots (77 kg sailor) with a 7.5 m2 Retro.
For this type of sailing (flatter water) I much preferred the Isonic 115, but when it got a little windier and rougher, I sailed the S-Type 115 quite a bit as well.
Hope this helps,