Do you have room in the tank to build a large enough trap to hold him? At that age he must be what, 12 inches? I've used egg crate to block off sections of the tank, gradually coaxing larger fish into smaller and smaller sections of the tank, sometimes over a number of days, eventually getting him into a relatively small corner, Then I've used clear plastics, like 2 liter soda bottles with a favorite food inside to lure the fish. I'm trying to visualize what size trap will be needed for yours and whether you have a DIY trap option or if a commercial option might be available.Depending on the rock formations, the rigid egg crate may not work, another possibility would be the 1/4" clear screen material used to construct screen tops for aquariums. It could be draped around the contour of the rocks to form a barrier from the bottom to the top of the tank. If you can gradually corner the fish into a small enough area, you may even be able to use a large dip net rather than a trap.
If you know where he "sleeps" wait until the lights have been off for a few hours. Turn the tank lights on.....it stuns the fish for about 15 seconds to the point where you can just reach in and grab them. Obviously doesn't work if he buries himself in the rock.
Theyre easiest to capture at night unless he sleeps deep in the rock. If not make something out of clear material to trap him or cut off sections of the tank to get him somewhere easier to catch. I dont think any fish traps on the market are big enough for a fish his size.
We had to remove a 19 year old sailfin from ours not too long ago. I used tenax plastic fencing to keep him on one side of the tank (had to maneuver it around coral and rock) but finally got him after I could keep him on one side