You have had some interesting advice in this thread, and I am certainly not going to argue against the advice to encourage him to try sports and martial arts, etc. to build confidence, as I love sports.
That said, your words "he loves cars" reminds me of one of the most important people from my childhood. My grandfather, who was born in 1908 and died in 1992 (I am turning 52 soon), was an incredible guy who loved cars, built his own house in the 1930s and drove a crane in a steel mill for 50+ years. My Mom had us spend time with our grandparents constantly while we were kids.
I was (and honestly still am) a pretty non-aggressive person, but my grandparents realized very early on that I had an interest in family history and history. They constantly told me stories about our family and told me all about things like the U.S. Civil War, the settling of the American West, World War I and World War II, the Great Depression and FDR's New Deal, and John F. Kennedy. What started as a basic childhood interest became my college major (American History and Political Science), and I finished undergrad in 2.5 semesters., as I could crank out history papers. I went on to get my doctorate (in business, but my dissertation was a business history topic) and I am a professor today.
While this is a nice story of how people can influence a kid's interests/career at a young age, I can honestly say that the greatest lessons my grandparents taught me were simply from spending time with them. They were amazing to our family and constantly involved with our relatives, and I got so much from being around them. I still see my Pap as the kind of guy I want to be when I "grow up."
If you have a friend or family member who is great to be around and maybe enjoys cars, this could be a life-changing person for your son. And, the best part about it is that it can just happen naturally.