Weapon Master is basically a cross between a Monk and a Fighter or Ranger. If you don't already have the class details, get them and read through them.
It has a limited choice of bonus feats and a slightly slower base attack bonus progression, unlike the fighter, but it does get a class feature or bonus feat at every level for a long time. For best results, the Weapon Master isn't going to be in heavy armor. I think Light armor is about as good as it gets before a weapon master loses access to class abilities.
There's one class feature in there called "Avoid damage" or "mitigate damage" or something like that. Basically, it takes a limited amount of lethal damage done to the Weapon Master and turns it into subdual damage. 3? x Wisdom modifier x (level / 2) hit point damage per day, IIRC. At 6th level, a Weapon Master (wis 12) can turn 9? points of damage into subdual damage every day. The choice to use it has to be made at the time the damage is taken, but the entire amount doesn't have to be used at the same time though. Think of a weaker version of the Paladin's "lay on hands" ability. It's a class feature found at 6th level. It's definitely something you have to look at and potentially adjust.
Of course, there is one major balancing point between the Fighter and the Weapon Master that doesn't exist in a the core rules; action points. A Fighter in Black Company gets more action points than any other class per level. Also, Command is a class skill for BC Fighters and a cross-class skill for Weapon Masters; another skill not found in a normal D&D game.
Finally, ask your player why he wants to use the Weapon Master class. If he's evasive about answering, tell him he can't use the class; odds are there's something about it that he plans to exploit ruthlessly later on.
