Try one of these methods that are used to catch fish.
Use bait to entice him better.
I posted this on another thread about catching a damsel. I hope it is helpful:
I don't know if it will work - I haven't actually put fish in my tank yet, but I read this on another forum, and also in one of my books. Because the thread I read it on is not remembered by me now, here is verbatim what the book describes:
The New Marine Aquarium Step - by - Step setup and Stocking Guide by Micheal S. Paletta says:
Because live rock makes catching fishes difficult, strategies have been devised to facilitate capture from tanks containing more than minimal aquascaping. In the early stages of a disease (which is the section I found this in) while the fish is still interested in food it may be possible to scoop it out with a large net as it feeds at the top of the tank. To do this requires some dexterity and luck - try to net the fish on the first attempt or it will become spooked.
Alternatively, rather than trying to net, a large plastic tube can be fashioned. An empty plastic soda bottle that has been thoughly washed and has it's bottom cut off is a good choice. The tunnel like bottle (with it's cap on) is placed on the bottom of the tank on its side and the infected fish is shooed into it, then quickly lifted out. To campuflage the bottome, it may be necessary to put substrate on the bottom of it and place live rock around the entrance.
If this fails, try watching where the fish rests at night and then either lift ut this pice of live rock with the fish still in it, or shine a bright flashlight onto the fish to stun it then scoop it out. Be sure to place the fish first in a ready bucket of water it has come from, then acclimate it to the water of the quarantine tank.
