If he uses hearing aids, the easiest thing to do would be to give him full-enclosure headphones like these: http://www.guitarcenter.com/KRK-KNS-...AID=728338210\
"Studio-reference" sound quality may be a bit much considering the hearing loss, but they should have the "oomph" he'll need.
May I ask what your IEM system is? Are you using a personal monitor mixer system like an Aviom or Hear Back, or are you using aux-fed monitor returns to headphone amps? Either way, he'll need his own volume control (so nobody else has to suffer), a "fuller" mix than you might give everyone else, and he may possibly need a beefier amp for the headphones (though IEM bodypacks are designed for high-SPL situations so they should be able to produce painfully-loud volumes in most headphones)
You'll also need to understand that hearing damage happens at high SPLs, no matter how much damage has already occurred. This creates a vicious cycle where you damage your hearing, can't hear, turn up, and damage your hearing more. It's just something to consider when putting loud headphones on a near-deaf guy and giving him the volume control; while it will certainly let him hear for a while, you may end up with a completely deaf guy.