Your concern is legitimate. Is he impulsive about other things as well? Does he know right from wrong? Does he break any other family and/or societal rules? Does he feel remorse after stealing? Has he ever returned things he has stolen? Is there a family history of obsessive-compulsiveness?
So much information is needed to help him help himself. So, I would suggest you consult a psychologist who specializes in working with children and families so you can address this issue and get to the root of the problem. Timely intervention will help him with emotional, psycho-social growth and prevent any future legal problems.