If you're the one doing the staring, a simple smile can go a long way in showing that you want to engage with him and are willing to start a conversation. It can also help to ease his anxiety around making the first move or expressing himself in social situations.
Cultural and social norms may also influence his behavior. For instance, in some cultures, it's considered impolite to make eye contact with strangers. He might be following this unwritten rule to respect your personal space or avoid offending you by coming too close. He might even be unsure whether you're someone he knows or not.
Another reason why he doesn't smile is because he's intimidated by you or feels like you're out of his league. It's common for people to feel anxious in unfamiliar social situations, and he might be afraid of the rejection that could come with approaching you.
He might also be a natural introvert who finds it challenging to initiate conversations. If he finds you interesting, it's possible that he'd love to get to know you better but feels too shy to approach you.
He might be distracted or preoccupied with his thoughts and experiences. Alternatively, he might be having a bad day and finds no pleasure in anything he sees. He could even be sick of everyone and everything. He is staring at you, but he doesn't smile because it would make him seem rude or discourteous.