Surgical wounds , specially of bones take longer time to heal.The person is given pain killers after surgery for few days lasting up to 2 months or so & pain killers are of opium group which is addictive but routine pain killers do not work after bone surgery. From outside it is a clean line. Entire sternum bone is cut to reach the heart ( unless it is the endoscopic surgery ) Get him checked for some infection on the wound area ( yes it can happen after 3 months also) Hope that He is Not Diabetic or Asthmatic.
Next Steps
Do Not Over Do Exercise of any kind whether it is physiotherapy or simple walks.
Health Tips
Time is a healer. increase the exercise gradually. Get a Simple X ray chest done + blood test for infection and
sugar levels