Moserr,

There are no exercises that will stabilize the sternoclavicular joint directly. It does not rely on dynamic stabilization like the GH joint. The muscles that attach to the area do not cross the joint, but rather pull it in their respective directions: subclavius pulls it down, SCM pulls up, clavicular portion of the pectoralis will have varied angles of pull, trapezius will elevate the lateral end of the clavicle and cause compression at the manubrial notch while pulling posterior on the clavicle. The sternoclavicular joint will continue to "do its thing" until the passive stabilization is restored via either a surgical reconstruction or regenerative injection therapy. If it is not causing pain, your main goal should be to work on the scapular stabilizers. Get his rhomboids and lower traps firing properly. This is of course only my opinion. It would be best to consult his ortho.

The Forgotten Joint: Sternoclavicular Dislocation, Subluxation, and Treatment