I'm not sure what exactly caused the heart attack in the first place. If the biggest blockage was 40%, then this is inadequate to cause such an event.
Does he have any muscle damage? Did they explain the cause of his heart attack?
In most cases, cardiologists don't treat a blockage with stents until they reach 70%. In your father's case, they would most likely treat him medically to help stabilize the blockages as there is no drug that will eliminate the blockage. There is some evidence that statin use along with aerobic exercise may produce a small decrease in the size of the blockage, however it is not really very significant. The goal will be to keep these from getting larger, they can do that with meds and lifestyle changes. What meds depends on things like his cholesterol levels and heart function. Do you know what his lipid profile was or his EF%, that would help.
Your father should work closely with his doctor to develop a lifestyle and medical treatment to keep his arteries healthy. Remember, one should never start an exercise routine without running it past their doctor first, especially some one with heart disease.
Jon