I taught 12 year olds for over 20 years, and it is the absolute worst time for kids to have organizational issues. The best thing to do is for you and him to come up with a check list that gets checked off before he's allowed to come home, and if he has a locker at school, tell him that he has to bring all of his things home every night. That way, you can look through things together and determine assignments that need to be completed and such. Also, if his teachers will agree, get a day planner and have each teacher sign it at the end of class. Have him write down tests coming up, assignments, homework, etc, and have the teacher sign off on it letting you know of any particular issues that come up. If it's not signed by all of his teachers, he doesn't get to do whatever he likes in the afternoon.

It is possible he is gifted. It is common for very intelligent kids to get bored and stop working. It is too early, in my opinion, to conclusively label him with anything. Sometimes a system to help with organization and a system of reward/punishment if he doesn't follow through is all you'll need.