OP, I think it is important to know whether your son is a threat to win any of these events (or all)? Also, is he a threat to qualify for States in any of these events (or all)?
My daughter just did this triple at Regionals last weekend and was fine, but she knew she was not going to win any of the three regardless (solid D1 P5 commit senior ahead of her in 1600 and 3200 and 800 is the weaker event of the 3). She was able to qualify in the 1600 and 3200 and score in the 800.
If she would have been going for the win in any of the 3, her coach would have just told her to stay out of the 800 and focus on the longer races. She was not worried about missing a qualifier in the 1600 and 3200 unless she had a total blowup, which she did not. I guess she didn't need to run the 800 but she gave her team some points and that is why they're out there, right?
At the end of the day, have him stay out of the sun, hydrate, and he'll be fine. It is roughly 5k of racing. Not a marathon. Stay loose, jog in between, a few hard strides before/after each race, stay in the shade, and don't eat anything stupid (no big subs after the 1600, for instance). Stick to a good sports drink and bananas, etc. This is when the old school Powerbars reigned supreme, LOL!!!! :)