On topic...
540 PTO is what almost all US implement are set to run at for input speed. There are implements in the higher (larger) classes Category 2 and 3 that use 1000 rpm. The PTO shaft is a different diameter so guys like your FIL have a harder time being stupi----eh ---- shall we say, "uncautious"? In asia and other areas they don't have the anti-stupid stuff built in (they have fewer lawyers), they trust the owners to be smart enough to figure out which implements are operated at what RPM.
You can move the lever to the 1000 rpm and operate the engine at 1/2 the speed that gives 540 rpm. For instance, if the engine give 540 at 2600 rpm, if you move the lever to 1000 and operate the engine at 1300 the PTO will give about the same speed as the 540 setting. Why do it? to save fuel if you are using a light duty low HP implement.
A rough cut mower at 650-700 rpm will give a better cut than normal 540 rpm. BUT - stuff can fly out of the cutter and go much farther and be more deadly. The gears in most implements can't take the higher speeds and so you don't want to go nuts. The potential for "bad things" followed closely by "loud expensive noises" is much higer at 1000 pto rpm, so keep that in mind!
I like to engage the clutch with the engine at 1200 rpm or so then flip the PTO lever and slowly let up the clutch peddle to feather in the implement. High rpm instant engagement is hard on the implement and tractor.
My advice is to enjoy the tractor, but buy a folding chair and a plastic steering wheel. Let the FIL sit in the chair, play with the wheel and make tractor noises, but don't let him on your machine! ;-)
jb