I hope he uses a kryptonite condom.
Which begs the question, since Superman is inherently infinitely stronger and more powerful than mere humans, which he hides in his everyday life, would not sex with him [or his son] qualify as incredibly dangerous?
IIRC, Larry Niven wrote about Superman procreating. His sperm would be super energetic and would rip apart a human female. Assuming he could fertilize an egg, the developing fetus would kick apart a human female. The answer was for Superman to carry the baby. (Good luck doing a C-section on his invulnerable skin.)