I would support your not purchasing guns as toys but I would not worry about his utilizing other play objects as guns. You cannot totally prevent him from observing the use of guns in the print and visual media. His peers will also be imitating the gunplay they see on TV, in movies, and in video games. To be "one of the gang", he will join them in their imaginary gunplay.
I would be far more concerned about teaching him tolerance for diversity and resolving conflict through non-violent means than I would about his occasional fantasy gunplay. I would also suggest limiting his exposure to and involvement in violent programming and games. Express and model your values regarding getting along with others. I played with stick guns and toy guns as did your husband. I'd like to think I turned out OK too. We need to provide our sons with ways to be boys and men that do not demand their being aggressive, physically powerful, and dominant. Thanks for writing.