Hi. Only when a person doubts himself he will start doubting others. Or if his expectations are not fulfilled he may seek clarity. So he may doubt. If generally he overcome on false things or people who have cheated him then he show doubtfulness . If he feels u r not true or u may not be try then also he may doubt. Like these there are various other conditions too. Try to bare with him and have detailed discussion where u suggest him not to doubt. Also if the condition exists you can meet a psychologist where they will help u.