It's better than sucking his thumb. It's probably just a little habit. You might try replacing it with a less obvious habit. If you think he's old enough, you could give him a ring to wear and teach him to twist the ring instead. He will probably grow up to be one of those who jingles his keys in his pocket. It just means he's high energy. It is probably a sign of a higher IQ as well.
I wouldn't worry about it. Peer pressure will take care of it eventually.
AFTER READING THE OTHER RESPONSES, LET ME ADD THE DEFINITION OF MENTAL ILLNESS (ALL KINDS) INCLUDES THAT THEY ARE SEVERE ENOUGH TO INTERFERE WITH DAILY LIFE FUNCTIONS. A DOCTOR WOULD NOT TREAT HIM FOR THIS UNLESS IT GETS TO THE POINT WHERE HE IS SO TEASED THAT HE WON'T GO TO SCHOOL, THAT THERE'S SEVERE EMOTIONAL DISTRESS, ETC.
Good Luck
D. Kimbriel
Grandma to 2 beautiful boys (One who twists his hair and one who pushes on his ears)