A mass was found in my fathers brain xray? Should automatically assume it is a brain tumor or is there something less serious it could be?

Don't assume. Most brain masses are not cancer. There are many benign (not cancer) causes of a mass in the brain. If your doctor said the mass has characteristics highly suggestive of cancer then you could assume it is until you have information otherwise.
Find out. Most brain masses aren't cancer, some benign brain tumors can be deadly as well (not much room in the skull for extra stuff to grow without squishing brain tissue). Some masses: infection. Benign tumors often surgically removed and not come back-can't usually say that about cancer (some of those can be treated with radiation). Make sure mass isn't spread of tumor or infection from elsewhere.