When a bog spavin is present, it is a sign of some underlying concern. If this horse is working hard, it may be a sign of joint strain. Also, if his hind end conformation is somewhat straight, this will put more strain on his hocks. Finally, he may have a bony/cartilage defect (OCD lesion) in the hock that may require surgery.
Treatment is determined once the cause is identified. If he is lame, or you want to determine the reason for the bog spavin, have your veterinarian examine and radiograph your horse's hocks.