You and Tom have been dating for months. He fell in love with you because you weren't afraid of him like everyone else was. Thats why his boggart is you being afraid of him.

He was in Lupins class and he got called to open the wardrobe to demonstrate boggarts. When he opened the door he saw you. You weren't wearing the necklace he got for you, and you were looking up at him in fear with tears. His heart broke. He yelled,'Riddikulus!' And the boggart disappeared. He quickly ran to your dorm.