You're wrong. He's a resident of Illinois, From Wikipedia:
"To legally possess or purchase firearms or ammunition, Illinois residents must have a Firearm Owner's Identification (FOID) card, which is issued by the Illinois State Police.[4] The police must issue FOID cards to eligible applicants. An applicant is disqualified if he or she has been convicted of a felony or an act of domestic violence, is the subject of an order of protection, has been convicted of assault or battery or been a patient in a mental ins ution within the last five years, has been adjudicated as a mental defective, or is an illegal immigrant.[5] Applicants under the age of 21 must have the written consent of a parent or legal guardian who is also legally able to possess firearms.[6]"
The bolded is key. It's also not abnormal for a kid to have a firearms certificate. I got one at 13, in Canada, through Cadets... (that does date me a bit, I'll admit).