For the OP, there are many ways to lead. When you have a choice, it's helpful to start with an interest or activity about which you are passionate. Music, a sport, kids, animals, veterans, politics, agriculture, language, journalism, religion, art, etc. There is no "better" or "worse" activity for SA purposes (at least for USNA) provided it's something productive and legal.
At his age, I recommend first just getting involved. You can help him find groups in or out of school that support his chosen area(s). For example, if he wants to help veterans, start with the local VFW or similar groups and work from there. Ask to meet with one of the group's leaders and demonstrate interest. Attend meetings and see what they're doing. Go from there. If he's into a sport, think of what he could he do other than play? For example, I worked with a (successful candidate) lacrosse player who worked after school and during the summer at the local lacrosse store (yep, the town of 15,000 has one). He could teach the sport at a summer camp for younger kids.
Once he's in the group (or groups), he can look for opportunities to lead. Start small and build from there. More ideas . . .
I worked with a (successful candidate) who organized her own tutoring program. Arranged with a public library to hold the sessions, recruited folks from her school to do it, planned the events, recruited underprivileged kids from her community to attend, etc. Did this every Saturday for 3 hours and during the summer.
Say he likes a particular breed of dog. Find the rescue group. They often participate in local events (dog fairs, etc.). He could work his way up to running one of those events. Or could start a dog-walking program for shut-ins.
The possibilities are endless and there is no one-size-fits-all.
What is less likely to be successful is to say: "What can I do that will look good for X college?" That makes it a job, not a passion and you're usually more successful with things about which you're passionate.
Finally -- if you lead in something about which you're passionate and a school doesn't like it . . . maybe that's not the right school for you.