Your father-in-law should be seen by a healthcare professional as soon as possible to determine the cause of the behaviors you mention.  Concerning symptoms might include memory troubles, confusion, mood swings, changes in personality, difficulty completing certain tasks, and difficulty finding the correct words.  You stated he does not have headaches or pain; however, his symptoms still indicate the need for medical attention.  
 
Your father-in-law needs to have a formal assessment in order to determine what is causing his change in behavior.  Typically, this type of assessment includes testing memory and thinking skills by doing a verbal interview.  This interview allows the doctor to determine if there are any symptoms of dementia, such as memory loss or disorientation.  The doctor will then conduct a physical examination and draw blood for testing.  The physical examination is used to decide if there are any medical problems that are currently affecting your father-in-law’s ability to think correctly, such as brain tumors, brain injuries, stroke or even dehydration.  The blood test is used to assess for several potential problems including medication side-effects, infections, a hormone imbalance, and malnutrition.  The doctor may also recommend seeing a neurologist for a brain imaging such as an MRI or CT scan. All of these tests help diagnosis probable Alzheimer's disease or other conditions contributing to memory loss, some of which may be reversible.