I was reading an article  that discussed how it could be more beneficial for sperm quality for men to wear kilts. I did find the suggestion funny, couldn't imagine my dh agreeing to it. Although, it has been found that lower temperatures in the testicle area are more beneficial to sperm. It would be a good step to make sure that your partner's "goods" are kept cool.
Have you suggested that your partner switch his brand of underwear to something less fitting?