If he keep stare at some pretty girls in the street, you can remind him that he needs to watch his steps, and ask him "honey, what are you looking at? Is there any interesting stuff around?". Or you can make some complements to those pretty girl's appetence, and tell him you know he is paying extra interest in those girls. If you continue to correct his behavior in an easy way, then maybe he will be embarrassed for his improper manner soon or later.
Good luck