It looks to me like he is trying to gauge interest in NYC through this email; as you can imagine, there is a lot of competition in the big city. Taking a train from Philadelphia to NYC and then getting a room at a decent hotel, maybe a meal or two, is an expensive exercise (he is a college kid after all...I remember when I was a broke college kid!) - not sure if he is really requiring a deposit, but all things considered I am not surprised that he is trying to limit his 'down side'. Service providers are not the only ones who can be flaky - their customers can do the same and cancel at the last minute, which will leave him out of pocket... My $0.02.
I would not pay a deposit - not something I do as a rule, but if he makes it to NYC I would be happy to see him!