* Super-Shaving: He could use his super strength to hold a razor perfectly still while flying at super speed, allowing the razor to glide over his skin in an instant.
* Heat Vision Shave: He could use his heat vision to create a precise, super-heated razor-like beam to shave his face. (Though this might be a bit risky!)
* Kryptonian Technology: Maybe he has a high-tech, kryptonian shaver that uses some sort of advanced technology to quickly and efficiently remove hair.
* Kryptonian Physiology: Maybe Kryptonians simply don't grow facial hair at all! This would be a convenient superpower.
Ultimately, how Superman shaves is left up to the imagination!