That is a great picture, by the way!
The best way to keep his tongue wet would probably be to increase the overall humidity of his surroundings. Does he live outside, as shown in the picture? Although mine's inside, my iguana's enclosure is really bad for holding humidity in. I don't think iguanas really recognize standing water as "water I can drink/bathe in". When I try to put Bud into his bath after I turn off the faucet, he clings to my arm. But recently I've discovered that he will jump off my arm into the tub if I leave the faucet running!
Maybe if you run a hose on low into the pool and let it fill by itself? I guess you'll have to be really careful about overflows, though...
Today I bought "The Little Dripper" from PetSmart, and it drips water into a big casserole dish inside his cage. I am hoping this will bring his attention to the water more... I also considered getting a bubbling water bowl from the same store- the girl there said her water dragon loves to lie on top of the bubbler in water container.
There's also a sonic mister that you can put in a waterfall or water container that creates a low-lying fog. It was about $40. The dripper bucket with hose and nozzle was about $7.
I also bought a pressure-pump gardening mister from Wal-Mart for $6. It sprays a fine, continuous mist without me having to kill my hand spritz a regular spray bottle. It's kind of like an airbrush machine. You just build up the pressure in the container by pumping the pump at the top, then depress the sprayer. This might annoy him less- it's a very fine mist. Or you can adjust it to a stream.
I notice that Bud will readily drink from a narrow stream from the spray bottle, if he's thirsty. I can tell he's thristy because he kind of smacks his lips. Other times, I'll try to get him to drink, and he'll give me "that look", like you say, as if he's never seen the spray bottle before, and would I please stop spraying his face, for goodness' sake!
Good luck with your iguana... he's very handsome. 