If he really seems interested, then it’s probably best not to second guess the situation; you should just assume he is being genuine and go with the flow.
However, it isn’t an entirely uncommon situation that the first date goes really well, but things cool down a little bit after that.
If he appears to be hanging on your every word and is genuinely interested in what you are telling him, then that’s obviously pretty positive.
If he is smiling a lot, and asking you plenty of questions without being evasive about anything you ask him, then that is also very promising.
Under such circumstances it can be tough to know just what his feelings towards you are.