You’d be surprised how many people don’t understand that… if you really have that box checked, find something he likes doing and take it to the next level. He is going to spend HOURS at it so, liking it is important.
The right answer depends on his target schools. CalTech and Stanford are going to want to see something really exceptional (and perhaps an Olympic medal). UCs will look primarily at grades/courses/test scores as a firm threshold then ECs to pick through the boarder-line cases. Neither CalPoly gives ECs much weight at all. A school like Santa Clara takes a more balanced approach and looks for well rounded students.
That said, FIRST is a very good framework and is well respected across the board - coaching a middle school FIRST Lego team while participating on his high school robotics team is pretty solid. Managing the web/social for grandma’s quilt guild is also solid experience and a demonstrated interest.
Encourage him to focus on the academics first, then follow his heart on the ECs. The enthusiasm and commitment will shine through.
I’d also note, there are less selective schools with very credible CS programs.
Good luck.