hi there, just wanted to let you know I have the exact same thing going on at my house.My son is 7 and very into battles and fighting etc.Although we do not have "toy guns" he makes them out of lego,sticks,pens etc.He has a very creative imagination.He pretends there are bombs going off and bad guys everywhere and is always battling with miniture army men.As long as he knows the boundaries socially (which sounds like your son does).This type of play is also good for the imagination and strategy building skills.I sit down with my son and play these games to both moniter and educate him in this type of play,afterall-his interests are important to him and valuable to his self worth.As long as they know the difference between games and real life.

my little brother is 6 and we had problems like that before. the whole thing was his parents used to let him sit in front of bad cartoons all day. and play the bad games. we finally got to the point of taking all the fighting ones away. maybe if u try that and avoid sections that have guns/knives etc. maybe by not being around or playing with them he'll get used to other things