Put a little birdy bath in his cage. That way, he can go check it out as he pleases when no one is around. If he seems to like that, you can introduce the sink or a shower.
I introduced my birds to water differently though. I put them in the sink without water in it. Once they were comfortable in the sink, I turned on the tap only so a little stream of water came out. They love to play with the little stream! You could try that too.
