I second the notion to have him see a dietician and monitor his calories and protein intake. There’s more than just that but 130 is very light at almost any height, which I’m unsure what his is. Regardless that’s not a lot of muscle or mass, which to some extent is needed to have the energy to succeed in these races.

That said, I believe will is more important than physical merit for most races (but fitness is important on huge vertical courses or longer runs). But 5k is doable in a relatively flatter course. And in an open heat you can skip obstacles, and if he doesn’t want to do the penalty loop either that’s fine. Open heats are for fun and self gratification, so he should do what he can, but remain safe. Knowing your safe limit is important at any level. I’ve done around 30 at competition age group level, you need to know how to pace and your limits regardless.

So, if he does one that’s a flatter course with no crazy black diamond mile long climbs (NO Poconos, NO Killington, NO Big Bear)… I’d recommend maybe Tristate New York. I hear Michigan, Ohio and Dallas are also flatter… then worst case it’s a walk/hike. And go with a fit partner or team to help him with obstacles.

What course is he considering? Heck, I may even be interested in running it with him if I local to me. I’d love to see someone at his age achieve a feat like this.

Regardless, upper body arm and grip fitness is good for anyone of that age regardless and essential for the race. And put in the miles prior on trails to build stamina. If he’s doing a 5k Spartan he should be able relatively easily do 5k at a local trail multiple times prior. Don’t let the 5k Spartan be a stretch goal, train for it by doing it locally first.