On some guns (depending on what you spend) there is a way to turn the pressure down, but it doesn't necessarily eliminate bruising..... just makes it less "damaging". I have a paintball gun (two actually), and enjoy the game VERY much. We play in my brother's woods, so I can't speak to the "paintball parks", but I can tell you that it is an adult's game.
I would say, it REALLY depends on your son. It stings when you get hit, and I've played with kids.... some who are fun to play with and can follow the rules and "take the hits". I've played with others though who can't take the pain, and end up in tears..... also, its EXTREMELY important that they understand the risks..... for example, we have strict rules prohibiting head shots and close range shots.... this is very important because of the nature of the equipment. If he can't understand how important this is and abide by the rules, then I suggest he waits until he can. Does he want it for a particular reason?? Does he have a forum to play in, or is it just for target practice / fun??
To be honest, I don't think kids under 16 are allowed to play in public forums here in Minnesota.... at least they wanted to make that a law..... I know they can't purchase the equipment (at least at the places I've been).
Oh, and one more thing.... the cheaper guns (Walmart specials) shoot slower, softer and less accurately, so if you decide to go with it, that might be the place to start.