I'm not sure this is always true or particularly healthy thinking. My DH earned a bachelors degree in a social science field then played professional soccer for a couple of years. He made so little money playing soccer that he couldn't even qualify for an apartment with two teammates. So, the leasing manager, liking his personality, hustle, and attitude, offered him a part-time job renting apartments to supplement his income and get him into an apartment. He is now the president of a property management company. (And a few years ago, he hired that same leasing manager into his Marketing department after she had been at home raising kids for many years.) I would say that at least as many people 'fall into something because it's there' as those who have a clearly-defined career trajectory.
Well, DH is a master plumber, who also does general contracting and property management, so yeah, I know that is a way to a decent living. It's not really "lucrative" that I mean, though; it's clear-cut. If a person goes to college and studies a specific course, they are on a trajectory to do something clear and actionable. They don't just evolve into a property manager or a dog trainer or a pizza restaurant manager. This is the part I don't like.
Even with DH - he didn't really sit down one day and decide his fondest desire was to be a plumber. He was at CC, studying computers (back when computers still used punchcards and took up a whole room), and he hated it and said, "well, I've been helping my dad with plumbing jobs, so might as well just do that." Does anyone set out to be a plumber because they think it's a fascinating business? Or do they just do it because it's available and an option that exists?
I don't want him to just fall into something because it is there.
Keep an open mind about your DS's options. He may turn out to be much more successful than a kid who aced his standardized tests. Success has a lot to do with grit, drive, resourcefulness; maybe much more to do with these things than pure academic abilities as narrowly-defined by a high-stakes (timed!) test like the ACT. Keep your chin up so you can help him keep his up!