OK, now that I know that they are for a Glock, I'd say get him one that reflects his personality/interests, if available.
Something with his initals might be good.
Avoid skulls and anything that might be interpreted as promoting violence, because, if the gun ever becomes evidence (and in a defensive shooting situation, it will become evidence) the other side will try and use the embellishment to "prove" he was actively desiring to shoot someone. A wanna be vigilante, or something along those lines.
They will be looking for anything and everything to try and cloud the issue, and things that say "die scumbag" to a jury should be avoided.