I have one who is also cow hocked. He was not as bad as yours at this age, but still was cow hocked. 2 yrs ago I free leased him to a therapeutic riding center, where a farrier, without my approval, decided he would remove some flares and try to slightly correct the angles. Within 2 months he could not even be used to therapeutic riding, was returned to me, and has been in full retirement ever since, with his legs even more rotated than before.
I would say that probably your farrier can help prevent it from progressing, but most likely, over the years it will get worse. Just based on my experience.
I will never let a farrier try to correct angles on a mature horse again, unless it is a slight heel angle or something similar. Certainly not when it torques joints that are mature.