You are correct in wondering if your son may be displaying difficulties in addition to Attention-Deficit/Hyperactivity Disorder (if indeed he is ultimately found to display this condition, which is manifested by an array of symptoms which revolve around three axes: inattention, psychomotor overactivity, and impulsivity). Temper dyscontrol of a serious, volatile sort is not a manifestation of ADHD, though problems with anger management are often associated with ADHD. By the way, very useful children's books re: management of anger are: Hot Stuff to Help Kids Chill Out (by Jerry Wilde, published by LGR Publishing in Richmond, Indiana) and How I Learned To Control My Temper (by Debbie Pincus). Re: ADHD, you might want to take a look at Taking Charge OF ADHD (by Russell Barkley). There are also a host of children's books dealing with ADHD. Good examples include Jumpin Jake Settles Down: A Workbook To Help Impulsive Children Learn To Think Before They Act (by Lawrence Shapiro), Shelley The Hyperactive Turtle (by Deborah Moss) and Puttine On The Brakes; Young People's Guide To Understanding Attention-Deficit/Hyperactivity Disorder (by Patricia Quinn and Judith Stern).
Relative to your son's stealing his sister's personal items, I would be reluctant to view this simply an ADHD-related manifestation of impulsivity. The nature of the items themselves, coupled with the fact that (by virtue of hiding them) he is aware that something in his behavior is amiss, indicates problematic functioning beyond impulsivity. You wouldn't be making a mistake by seeking child mental health evaluation. Don't let his age stand in your way. The younger children are when these problems are addressed, the better. Every week I see anywhere from 15-20 youngsters who are approximately your son's age. You needn't feel any reluctance in arranging an evaluation - you will be helping him.
This information is provided for general medical education purposes only. Please consult your physician for diagnostic and treatment options pertaining to your specific medical condition.
*Keyword: behavior problem, ADHD