Of course, it’s an EC. Valid and good.
The bigger question is whether a competitive college will find his math-sci experiences (other than classes) sufficient to choose him for their engineering program (whether or not it’s ‘direct admit.’) The more competitive the college is, the more applicants with the right patterns of experience they have. That’s what he’s up against. This isn’t about just filling out the Activities section, showing how your time was spent. Rather, showing them you’re ready, in various ways that specificall relate to stem/engineering.
This goes beyond character.
Posters here likely know of colleges that accept more kids without a background that includes engineering or math-sci ECs. It would be good to share those ideas.