Writing an impulsive, overly enthusiastic character. He's not stupid (he knows what he needs to do in the situation they're in) but he can be rather impulsive at times.
And I don't know how to write him without making him come off as an idiot, though.
Maybe have him do or say stupid shit and regret it bitterly afterward.
Ah, summer, what power you have to make us suffer and like it. ~Russel Baker