It might be becoz of his age r ftom the feeling of being isolated /neglected/no respect,r due to passing of age,fear of any diseases that may let him bedridden,or any medications if he is taking already,those chemicals also may become cause for short temper.If he is having Hypertension,once get him for  Check up.If u r sure he is quite healthy,then u try to b friendly,once take to any Psychologist for counselling r to know the reasons from his side.,Latr depending on this result,u may need to see Psychiatrist,if pathological.