Maybe its just a stage? I often find with my little brother (who is now 12, but im thinking of the last few years).. sometimes he'd be so vague or whatever... but its just because they are so involved with thinking about something else.. theyre imaginations and thought patterns can be so amazing and complex!

I guess if his grades and stuff arent affected, maybe dont worry about it too much.. You could try talking to him, in case hes thinking about something he thinks is bad... just let him know youre there.. but hes probably just normal..

K