Hi Nightflash,
There is no surefire thing that will make your young hibiscus bloom right away. Some varieties just take longer to start blooming than others. Take for instance Young at Heart, that CV needs to grow several feet before it starts to produce buds. On the other end of that spectrum, some CVs start to bloom soon after they are propagated, one such variety at least for me was blue ballerina. It bloomed when it was about 4 inches tall. All you can do now is just keep the plant watered, fertilized and give it plenty of sun and wait for it to do its thing