Since he's already got 80 models or so, telling him to scrap his army for another one might be a bit of a bad choice.
However, since we're probably going to be doing a bunch of friendly games rather than the tournament circuit, a bit of house-ruling might be workable.
Given the Night Lords fluff how does this sound:
Terror Tactics:
Units with this special rule gain the Stealth USR until they either shoot, manifest a power, or strike in melee. If they lose this stealth by shooting, then for that shooting phase their weapons gain the pinning special rule. If they lose this stealth by manifesting a power then any leadership check related to powers manifested that phase, or due to casualties inflicted by powers manifested that phase, are made at -1 Ld. If they lose this stealth during the close combat phase then they gain the fear special rule for that phase.
Terror tactics is possessed by Night Lords Chaos Space Marines, Chaos Terminators, Chaos Chosen, Chaos Lords, Chaos Havocs, Chaos Raptors, and Chaos Sorcerers.
Thoughts?