I have had luck with stimulating rain either by doing a large water change or by misting the tank. Try not to make the water the exact same temp either a few degrees off is fine. With some super picky non breeders I had luck with a method OFL mentioned. I kept my males in super tiny cups with no water changes for a week but kept feeding super good foods. Then I dumped them in without acclimating them at night. Any healthy fish will be able to withstand the shock. It also got me a really nice spawn. Another thing you can do is add another male floating in the tank. Sometimes at the presence of an "intruder" it might motivate him to spawn.