Graduation rates are typically reported for 6 years--so even schools with "excellent" graduation rates have a lot of students doing an extra year--it can be because of health issues or an accident, study abroad, violations, transfers, gap semesters, grades, double major requirements etc. etc. He will NOT be the only one and many people won't really have kept track of what year anyone is in. The people in his class who were freshman with him are likely the only ones who really remember that they are in the same year.