Your DS' 6th semester transcript is almost certainly required which wont be available until after end of school year.
Also some MOCs require specific letters of recommendation from certain teachers and administrators; many of these folks handle all requests [they get a lot from every college bound Senior] at beginning of school year. Doesn't hurt to check and ask now but some of DS' teachers deferred until start of school year.
There is no real advantage to doing this now IMHO; in fact some stuff might get misplaced if its there so early.
Work on the essays and practice for nomination board interviews over the summer; be ready to roll first day of school.
Make sure you have the codes for MOC office to send SAT/ACT scores; if no code leave a few weeks for snail mail.
Also, your DS should look for meaningful opportunities to fill his summer (job, volunteer service, etc.) and to bolster his leadership experience at the start of the school year (team captain, officer of a club, etc.). IMHO, I would suggest working on essays and other supporting materials over the summer, but wait until after the beginning of the school year to make sure his package is strong as possible when submitted.
Note: all three of my DS's MOCs had different submission requirements. Make sure your DS reviews requirements and deadlines for each potential nomination source.
What I would do now is just call your MOC (the future cadet needs to make this phone call) and ask when they start accepting applications. It's my understanding that you can't apply before the 'window to apply' opens. At least, that is the case in our state. The MOC had a very specific protocol to follow.
I don't know about your nominating sources, but I contacted Bob Corker's office and they said that even if I send my package in early, I could update my credentials and ACT scores until the deadline. Check and see what your sources have to say about it.
Every MOC is different. Start with their websites. If that doesn't provide the info you need, your child (not you) should contact the SA nominations person for each MOC and ask how early they start accepting applications and whether they will allow you to update the package once it's submitted. Some MOCs don't permit additions -- in that case, it may help to wait. If you can add new information until the deadline, getting it done and submitted now is one less thing to do in the fall.
Each MOC office usually has a staffer who acts as a service academy coordinator. I also agree that each office runs things differently. Contact the SA coordinator at your MOC's office and you will get the best info for that particular congressional representative. Be sure you review the congressional website first though.
I think people here have pretty much said everything... Definitely start with your Congress person's website, they all (should) have websites that aren't difficult to find. On the website, see if there is a specified "window" when the nomination period opens, or they begin giving out the nomination application. If you have to contact the office (calling or emailing) to get the nomination packet, make sure that your son is the one to reach out and talk to them, not you. He should be more than motivated enough to do it, and it will make a better first impression to the office. Good luck!