I agree this might be a good approach. Briefly mention all 3 but go on to say but one in particular stands out and impacted his decision to pursue a healthcare field.
In general, there are no “right” or “wrong” ways to handle these essays. Everyone will approach them differently. The goal is to give the admissions folks a better idea of who your son is beyond the list of stuff on his application. I think his approach of going beyond just listing the same activities already on his application is smart and might make his application more interesting to read than it otherwise would be.