How can I get my son to stop cracking his fingers?

Do You Need To? I know that this may be annoying, but I am not aware of any harm done by this habit. Since it tends be a release of " nervous energy", i worry that by supressing it, more complicated issues arise such as headaches or stomaches.
Depends on his age. Cracking joints is a habit for lots of people -- kids and adults. If he's young enough to appreciate rewards, each day that he goes without cracking his joints earns him a small prize plus a sticker on the calendar. A week of each day earning a sticker earns him a larger prize. After a few weeks, the habit should be gone. If he's a teen, he'll have to decide to stop on his own.