Just a reminder, if his application to the University of Maryland, College Park meets the early action deadline of November 1, then he would be automatically considered for merit scholarships, including the Banneker-Key, ranging from covering the out-of-state tuition to a full ride.
Your son has very strong stats and would stand out. I would not be concerned about his ECs being “some math/CS stuff, nothing exceptionally strong”, for a laundry list of ECs cannot beat his articulating what he has learned from these experiences (even from just one or two), in helping him to shape and build his characters, as well as broadening his outlook on life.