Is your dad taking other medications? Make sure to check his heart beat to make sure it doesn't go down further. Metoprolol tartrate can make a person feel tired.
The doctor discontinued the medication due to his opinion there are (common) side effects may include:
Depression, diarrhea, dizziness, itching, rash, shortness of breath, slow heartbeat, tiredness.
The chest pains are another issue and could be the result of restenosis of the previous treatment of angioplasty (balloon, stent?). The symptoms of chest pain, excessive sweating , etc. is consistent with ischemia (lack of blood flow) possibly with restenosis or other blockage. It would get an immediate checkup, if it is ischemia there can be permanent damage to the heart cells. Even if the pain goes away, that is not an indication everything is well, as there can be silent ischemia (no symptoms).