I'm a 21 year old male. I currently wear boxer briefs and occasionally briefs. My dad wears boxers. He has worn boxers since he was 17. My dad told me that I need to start wearing boxers all the time because it is healthier for my testicles and my sperm. He said I'm 21 and need to be thinking of my sperm health for when I get married and try to start producing children. Is he right?
No, he is incorrect about this. Years ago there was a study of workers in an iron smelting plant. It was discovered that the workers were less fertile because of the heat that they worked in, but that their fertility recovered after being away from the heat. But the amount of heat we are talking about is above 105 degrees and the length of exposure is on the order of hours.
Briefs do keep the genitals slightly warmer (just less than a degree) than boxers, but this is not nearly enough to change a person's fertility, unless you are outdoors in the desert heat all the time or sitting in a sauna for hours on end. For that matter, tight jeans would raise the testicle temperature more than briefs worn with loose pants, but still not enough to impact fertility unless you are in a hot environment.
For the vast majority of men, pick the style of underwear that you find the most comfortable to wear.