OK, I'm a bit unsure now. Are you saying he does qualify on high school education? Then I don't understand why he didn't fill in on the DS260 but all he needs to do is take his high school certificate along to the interview and he should be fine (as long as the schooling meets the requirement).
KCC cannot make any decisions on the validity of the application, only a consular officer can do that. So if someone has returned the forms, KCC will schedule an interview regardless of whether or not they can see any problems in the application.