bacdorslider,
My son is a month younger than yours. We are about the same place in this baseball lifecyle. Our team has about 6 pitchers that throw in the 80's, so your son is right where he needs to be. The important thing is getting him visibility in showcases like the Perfect Game WWBA tournaments and the fall Underclassman showcase. I would expect your summer team will be coming to these.
We have been to both the USA tryouts and Perfect Game tournaments the last 2 years and I would certainly pick PG over USA. PG offers a lot more visibility and opportunities to be seen. I am not saying don't go to the USA tryouts if an opportunity provides itself, but would not sweat it if you can't. He would be in the 16U trials and competing against guys that are up to a year and a half older than him (Jan 1994). It can be hit or miss on making the team. If he does not make the team, the visibility stops. At least with PG events, you have running updates in their database and can add to it as he goes to future events.
Those are really just two different types of events, with different types of expected results.
Back to your original question, how much should they throw. That has always been the debate. Leo Mazzone and I have talked about this several times. A lot depends on whether in-season or off-season. In season, they need to throw regularly, even if it is only 60% effort between starts.
I believe in the off-season, they should throw more to work on mechanics and not as much about velocity. Coming into pre-season, work on long tossing and developing the strength back. As they are really starting to build for the season to start, having a good bullpen routine is important. Leo always made it a point of saying "Long tossing is throwing a distance further than what your normal throw distance is. If you are a pitcher, it is some distance further than 60'6"." His main point is don't long toss at a different arm angle and approach than you will in your normal playing position. Long tossing high arching throws is not as beneficial as throwing something shorter, on a line.
My son's pitcing instructor has been working with him since he was 9. He is a 37 year old LH'er who can still hit 90-92 on the gun, (came up in the Yankees Org with Jeter). He says guys want to throw hard, but they don't want to "Throw hard". Meaning they don't want to work at throwing hard, they just want it to happen naturally. He recommends the same that Leo did, and that is use the last 10-12 pitches of each pen (after they have gotten really warm and lathered up) to dial the effort up 90-95%. Don't focus as much on the accuracy as how to get yourself to dial it up. The only time your effort reaches 90-95% can't be in a game when you are facing a tough team.
I know there are a lot of different philosophies on this subject, and there will be some to strongly disagree on this approach, but everyone has to figure out what they believe and what they feel is the best approach for their kid. Sometimes that is dependent on the kid, and sometimes that changes as you learn different techniques and approaches.
Either way, I hope he has a great season. Let us know if he will be down this way playing this summer.