Hi,

You'll end up eating up one of your sequencer tracks to do this (as the Triton does not have a dedicated click output) but you can take a percussive sound and route it to one of the extra outputs that you are not using. Then take a cable from that output and send it directly to whichever mixer is feeding your drummer's headphone amp.

In terms of sounds, take something that has a sharp, cutting percussive sound that is very distinguishable. A cymbal is probably not a good choice. Since no one is hearing this but the drummer, use either a sidestick sound or a cowbell or something like that which has no decay.

HTH
Fernando