Your father, if he wants to live a longer life, has to quit smoking completely. Believe me, as a former smoker, you can't cut down. I quit 25 years ago by simply sitting on my hands and hurting for a few days, and it got better quickly. Millions of us have quit. Keep in mind, every time your father smokes a pimp stick, his arteries will enflame, whether he smokes 15 or 50 pimp sticks. He has to face the fact that all his meds will be worthless unless he quits completely.
Backache and unusual stool might be caused by the long hours in hospital bed. It should subside very quickly.
Being easily tired will probably be the direct effect of the STEMI and the heart functioning now at 20%. This could improve over time also, especially when they place the 2nd stent.
Good luck to your father.