Hey dear...what can I say. Men...ten a penny
Well, personally I think that it's really cute when guys get all crazy and jealous (therefore revealing their insecurity), and besides...if you say you're pretty young (which I believe he is too) you should just let him slip away with that once because he's just immature. That doesn't mean you should forgive guys all the time, it just means that you can let him slip away once given that you guys will talk it over and he'll apologize. Next time he'll know, because you'll introduce him to something new - trust.