How do I tell if he's serious about me?

“Serious” can mean exclusivity, a future together or dependable contact. Unless both of you mean the same thing, evidence can look convincing while answering different questions. Stating what you want makes the conversation more precise.

Consistency between agreements and actions matters over time, especially when a plan changes or something goes wrong. It can help you assess the relationship being offered; it cannot give certainty about an unspoken inner state.