If the kid did take the gun from his parents then chances are high he probably didn't have to try very hard, at 15 years of age I would say chances are better than good that he either knew exactly where the key (or combination) for the safe where.
When I was 15 and lived in another country where guns are much more restricted, my dad had two shotguns and he taught me how to operate and how to get to them in an emergency. Different place, different times but concept probably remains, unless the parents knew their kid was disturbed and not be trusted chances are high at 15 he would know how to get to and operate those guns.