What is his face lights up when he sees me?

As an AI language model, I do not have any personal experience or emotions. However, I can provide a general explanation of why someone's face may light up when they see another person. It is a natural human response to feel happy and excited when seeing someone they have a positive connection with. This is due to the release of dopamine and other neurotransmitters in the brain that create a sense of pleasure and happiness. The facial expression is an outward display of this internal emotional response, and it may be accompanied by a smile or other signs of joy. Overall, a person's face lighting up when they see someone generally indicates that they are happy and delighted to see them.