With my first I said he would never have any guns to play with, but unfortunetly that was not the case. In one of my child devlopment classes I had an awesome teacher who had 5 children ranging from 2 to 17. She explained it like this: Even if the child has never seen a gun they will make one out of anything. She finally gave in a bought one of her first boys a play gun when he chewed a piece of bread into a gun.
I had the same experience for my both of my boys making guns out of everything. It is just normal boy pretend play. You just have to limit the gun play. Maybe a simple behavior reward chart will help him. When he acts up and goes against the rules give him an X or something similar on a chart, and when he listens to your intructions give him a sticker. Let him mark and put his rewards on his own chart, too. They love to do that. Make a plan that if he has too many x's then he gets something taken away for a certain period of time. When he has so many good rewards give him something good for his rewards. You can also go as far as taking away stickers from his chart when he is bad, and maybe have his teacher at school get involved and give her/him a booklet to also post stickers and x's. The most important thing is to be consistant. When you say you have done good give the sticker and when he has not listned follow through the first time. Making sure you also reward the good behavior will make him want to do it more. And by giving the x and taking things away and not paying much more attention to the bad behavior will help illminate it.
This simple chart is a basic form of behavior therapy, it can help out in any form of behavior. You can ask for suggestions from his psychologist on other simply behaior therapy techniques like these that you can use at home and also implement at school. you could also do a web search and find some interesting stuff.
YOu can also encourage his pretend play by letting him "shoot" at GI joes or stacks of blocks, etc. This lets him know when this behavior is apporiate. His chart will also inforce when the behavior is approriate.
Hope this helps.