Sarah, he's injecting his thigh, not his butt.

Jesse, as long as you're injecting lateral to the midline of the thigh, you should be fine. What size needle are you using? I use a 1", 25 ga. needle, which I think is smaller than what most docs prescribe. If your thighs are small, you probably don't need one that's any bigger than that. Hurts less with a smaller one, too.