Some men just take zinc that's the most common supplement. Zinc also increases male sex drive.
There are others too that OH's can take too, Zinc improves count, selenium improves motility, l-arginine improves quality and count, calcium is what helps create sperm in the first place, B-complex and C are recommended for general fertility
Also he needs to be taking these for at leat 3 months for them to have any effect.