First, as a teacher, I wish all my students had the wonderful family support system your son does.
The trick to this assignment is to understand that the experience does not need to be life changing in a huge and dramatic way. Instead, it's really designed as preparation for a certain type of personal statement in a college admission essay, about how a specific incident leads to greater understanding.
A typical event to choose is one in which your son learned a life lesson. For example, if your son had pets, could he describe how some incident with a pet taught him something about responsibility? Was there a moment when he was hanging out with classmates where something small gave him a greater understanding of the meaning of money (for example, suggesting a group of kids head out for ice cream after school, and discovering how embarrassed a classmate was at being unable to afford it?) Did he ever have a disabled friend or a gay friend and discover something about the nature of discrimination or fairness when his friend was bullied?
The best way to work on this assignment with your son is to have a conversation with him, asking him questions about whether he can remember any incidents that taught him lessons about life, or lessons about how to be a better person.