Bas3balldad,
Is pitching something your son is interested in? If so, and if you decide the proper instruction isn't going to be had from the HS coaches, then you might want to consider private instruction.
To answer your questions...
Bullpen sessions can be used to work on any number of things as BOF pointed out: different pitches, hitting spots, pitch sequencing, mechanical issues, etc. You might work on only one or two things if those things really need some work. Otherwise, you might touch on all of them. What your son should do will depend on what he needs to work in. It really just depends.
How many pitches should you throw? Again, it depends. It depends on when you threw last and when you'll throw again whether it be in a bullpen or in a game. It could be anywhere from 30-75 pitches. In preseason, you might throw fewer pitches to start with and then work your way up to the start of the season. When throwing larger numbers, you might consider throwing them in sets of, say, 15 with a short break in between to simulate innings of a game. These precautions will help avoid getting into a deficit situation early in the season.
Bullpens, IMHO, do not need to thrown entirely at 100%. But you can't expect to be successful throwing 100% in games without having done that in the bullpen especially since 100% at game time is higher than 100% in the bullpen due to adrenaline at game time.
Bullpen performance will factor into the coach's game time decisions though game time performance will often trump bullpen performance.
Last but not least, I think your son should change his thinking that throwing hard is the only way to impress the coach. Good control and good movement will also impress.