It's Scully that makes or breaks this. Get her heated flesh sliding along his, and his cock reacts. Any real effort on his part doesn't do a damned thing.

And that's how he ends up fully hard as she tweaks his nipple, making a satisfied little noise in her mouth. As soon as he's sure he can manage it, he abandons her clit in favor of guiding his cock to her entrance. Hurry, some instinct says, or it won't be possible. Now or never.