You have described a coronoid tip fracture of the elbow. In other words, the tip of the ulnar bone in the forearm has broken off. The elbow is a hinge joint made up of three bones. These include: 1) the humerus (the upper arm bone), 2) the ulna (the larger bone of the forearm, on the opposite side of the thumb), and 3) the radius (the smaller bone of the forearm on the same side as the thumb).
The end of the ulna at the elbow is called the coronoid process. It is shaped like a hook that fits around or hooks over the spool-shaped end of the humerus.
When a large fragment breaks off the coronid tip, joint dislocation is likely. Smaller fragments can also result in joint instability. The reason for this is because the joint capsule, a fibrous covering around the outside of the joint, attaches to the tip of the coronoid process.
If the tip is disrupted, the capsule is damaged too. Without the capsule to hold the joint together, dislocation can occur. According to a recent study of cadavers (human bodies preserved for scientific study), even small coronoid tip fractures can lead to elbow instability.
Surgical repair is the best way to prevent this from happening. The surgeon wires the bone fragments back together and holds everything in place until healing can occur. Your son may have a better chance of recovering full motion without further problems with this treatment plan.