The brand names are widely known by all players, ask around. The field MUST have a barrel plug rule and enforce it in the safe area, because players will have their paint markers in the safe area, where no one will be wearing goggles. The barrel plug is a device which prevents a paintball from leaving the barrel, should the paint marker be fired accidentally. The field MUST have competent and experienced field staff to enforce the rules of safety. Some fields have minimum ages.
Some fields require the parents to sign a permission slip for players under a certain age.
Ask for the name of the company that carries his insurance. Ask for a copy of their waiver form. Read it carefully, when you get home and be sure that the waiver only provides protection against players suing field owners for something stupid the player did. The waiver should show that if the player is injured through staff or owner negligence, the owner WILL be liable. Yes, this will decide what kind of day you are going to have, and perhaps what equipment you should bring along.