Most western headstalls should fit most Arabians. If he has a short head and you're on the last hole, or if you're using an English bridle, then it's best to get an Arabian headstall or Cob English bridle.
Here are some good Arabian western headstalls.
Saddles, Arabian Saddles, Western Saddles, Headstalls, Bits, Pads
Most normal bits will fit an Arab fine. If the bit sticks way out on either side, obviously too big, then look for one with a 4.75" or 4 3/4" mouth piece.