Reversible ischemia and generally indicates that a significant coronary artery blockage is present. Ischemia (lack of blood flow) to the heart can cause chest pain (angina).
Yes, mental stress can cause ischemia as well as phsyical exercise. If angina occurs with stress, physical exertion and there is relief from angina with rest it is referred to as stable angina, if angina is not relieved with rest it is referred to as unstable angina. If the chest pain is ischemia, nitro medication will provide relief.
Whether or not to drive a bus may be contraindicated until there is a diagnosis and treatment, but a doctor who has your husband's history and other information would be in the best position to advise.