Your son may have what's called a "trigger finger". Inflammation of the lining of the tendon keeps the tendon from sliding and gliding through its tunnel. It literally gets stuck.

It's also possible he has a locked metacarpophalangeal (MP) joint, which is the large knuckle on the back of the hand. There are many possible causes for a locked MP. Since he's so young, it's unlikely that degenerative changes from aging are the problem. It could be the shape of the bone. A large bump on one side or the other can catch the ligament and pin it against the joint.

Sometimes a piece of bone gets bumped off and floats inside the joint causing catching or locking. A torn ligament or rough edge inside the joint can have the same effect. There's no way to really know for sure without an X-ray and exam by an orthopedic surgeon.

Have it checked out if this is happening on a regular basis. It may be a quick fix if caught early on.