By three weeks after surgery, the wound should have been sufficiently healed as to not do that. Sometimes other health conditions, such as diabetes, can cause delayed healing of wounds. If that is the case with your father, the underlying health condition may need extra attention. I'm so sorry that happened.
This is very unusual, the sternum should have strong wire wrapped around it in at least 4 places. They also tell you to cuddle a soft pillow into your chest when coughing. They will now have to re-wire the sternum and stitch him up again.