In addition to the advice so far, @azar5065, he should make sure that at least 5% of his base pay is going into the TSP for the Dept of Defense match from the Blended Retirement System. (This is the default, but he needs to make sure it's happening.) His TSP account should be started after 60 days of service and his matching DoD BRS contributions will start on his 25th month of service.
Military compensation is so lightly taxed (compared to civilian compensation) that he should contribute to his Roth TSP and his Roth IRA... no traditional contributions are necessary from a tax perspective. This tax issue is so significant that he should keep up this habit until he reaches the ranks of E-7 or O-4.
The TSP's default investment is the longest-dated target retirement fund. If he wants to be more aggressive then he could invest in the C&S funds. If he wants to be very aggressive then he could invest in the I fund, which is one of the few TSP funds with lower expenses than most international index funds.
After he's set up these financial basics, he could check library copies of "The Military Guide To Financial Independence and Retirement", ChooseFI podcasts with military guests, and Spencer Reese's MilitaryMoneyManual podcast. However it's far more important that he get qualified and keep earning those promotions.
If he's told that he's a great candidate for submarine service then we should talk. You're both also welcome to contact me here (or e-mail NordsNords at Gmail) if you have more questions about anything military or personal finance.