Boxers or Briefs?
Sperm are stored "outside" the body for a reason. They can't tolerate excessive heat and the testicles' independent structure and considerable surface area keep them safe from the elevated core temperatures inside the body. Some people claim that Y-chromosome sperm (boys) are even less tolerant of heat than X-chromosome sperm. So, if you're trying for a boy, ask your mate to wear boxers for a while. This will keep things cool and give his "boys" some breathing room.
A cool bath or an ice pack to the scrotum would take matters further, but it's probably a rare male who would be that keen on producing a son.