Razix actually has two sex scenes with Yoshi, in Chapter 6 and Chapter 10.
To get the first scene, you'll have to have oral in Chapter 5, otherwise you get the oral scene in Chapter 6.
If your choices bypass that first time in Chapter 6, at the end of the story, you'll get that one instead of the second scene. And the first sex scene doesn't have a CG yet.
So to get the option to top, you have to have already have sex BUT also you told Razix you loved him when that option comes up.