Short answer: "More preload for more weight."
VERY generally speaking a road bike should have about 25-30mm of "sag" at the front, and 20-25mm at the rear, under load.
More specifically:
http://www.motorcyclenews.com/mcn/p.../products/productsresults/suspension/2006/november/feb23-05-set-suspension-sag/
The more aggressively you ride around corners the more important suspension settings are. If you ride like an old lady, puttering around and enjoying the scenery, it probably doesn't matter so much. Set it for "comfort." However, "comfort" doesn't necessarily mean "loose." You don't want to be bouncing up and down. That's not comfortable.
The BMW owner's manual is oddly devoid of information on this subject.
That link I provided has a very good step-by-step procedure for setting spring preload. So here's what I'd do:
1. Measure the fork "sag" under load, rider only with no luggage, and use that as a guideline. If it falls into the "25-30mm" range, then you can shoot for "20-25mm" at the rear. Adjust your rear suspension "target" according to what you get for the front.
2. Once you get the rider-only setting figured out (by counting the number of turns from fully "loose") you can set it up with a passenger aboard, aiming for the same amount of "sag" at the rear. Finally, figure out how many "turns" you need for a full load: Two-up with luggage. You should really only need to have 3 numbers in your head. When you add a passenger, simply crank in the required number of "turns". Then back off when you're riding solo.
Yes, you use the provided spanner to set the preload.
OR....you can just leave it where the dealer set it when you were about to ride away with your new bike, and don't worry about it. I don't change anything when I carry my daughter with me. I just don't ride aggressively enough for it to matter. It feels fine to me. If we go for a really LONG trip, with luggage, I'll just crank in 3 or 4 extra turns and see how it feels.
As for rebound: I have the ESA, so I don't have to worry about it. And "Comfort" works for me all the time. I never even mess with it. If you have manual adjustment, you just add more rebound in proportion to your preload....but that's all I have to say on THAT subject because I don't really have a clue.
Dude, if your son is a motorcycle racer why are you asking US?

You should be EXPLAINING it.