What is his face lights up when he sees me?

The expression "his face lights up when he sees me" is a colloquial way to describe a strong positive emotional reaction someone has when they see you. It suggests that the person's facial expression visibly changes to one of happiness, excitement, or joy. This reaction can include smiling, brightening of the eyes, and an overall look of enthusiasm or pleasure. It typically indicates that the person has affectionate or fond feelings toward you and enjoys your presence. This can occur in various types of relationships, whether romantic, familial, or friendships, and is usually a sign of warmth and connection between individuals.