Play fighting is one of the favorite games of the almost every young boy (and many girls) that I have met. Don't confuse it with real fighting. It is play, and the object is NOT to kill or hurt each other. This kind of play is not about being violent, it is about a host of other positive things being learned, including physical coordination, how to make physical contact without hurting someone, and how to control your physical and emotional impulses.
If you watch you may notice that almost all of these games are morality plays dealing with very heavy issues-good vs evil, the powerful and the powerless, life & death, compassion vs. retribution, etc. If you participate with them in this play, you will be able to help them explore these areas by introducing themes and morals that illustrate your own moral beliefs. This is morally, physically and psychologically very useful play, and I would encourage you to help your son explore these things.
In my experience, the kids that are dangerous and most likely to hurt someone are those that are not allowed to play this way at home. They don't have the experience to know how to play in this way without inadvertantly hurting someone. They don't have the physical or emotional control neccesary to handle the excitement!
So until you have had ample chance to help your child learn these kinds of controls by play fighting with him in the safer environment of your home, I would keep a close watch on his interaction with other more experienced players. Once he is comfortable with this kind of play, he should be able to have a lot of fun without getting hurt or hurting others, and learn a lot in the process.
Probably not what you wanted to hear, since I assume from your post that you have some kind of strong objection to play fighting
I do strongly feel that you are handicapping your son by not allowing him to explore these things with the people (his family) who can best make sure he learns them without getting hurt or hurting anyone else. And, you are missing a great opportunity to role play with him (and teach him your understandings) about all these very real and deep issues that center around life/death, good/evil and the many varieties of human conflict.