Don't be first, don't be last, volunteer for nothing, and keep your mouth shut!

Seriously tho...it's much better to be in shape and squared away than not; he doesn't want to be labeled a shitbag or placed in a workout platoon. Focus on 6 minute miles and sets of 10-15 pullups and 100 crunches every other day.
Living at altitude helps a little when running in San Diego or Pendleton. Perhaps try doing some 24/48-hr sleepless periods to get used to the feeling of fatigue and to see how he will react to them.
Any leg up in this area will be very helpful near the end of boot, during the Crucible. Try some hikes in the hills with a pack, or better yet, try cross country running in boots.

If he is interested in college at all, I suggest looking into the Delayed Entry Program. Don't let the recruiter keep sweating him to commit to something; he has a quota to fill.
Based on ASVAB scores, he might get his pick of MOS, but FWIW, it might not translate to a civilian job later on. Be forward thinking on the matter, since infantry skills are not really a job qualification these days.

Also, "choice of MOS" might translate to "choice of MOS available at units that need that MOS filled"

Good luck!