My Hens Keep Laying Infertile Eggs Even Though They Are With My Rooster. Is There Something I Can Give Him To Increase His Fertility? Or Is 8 Months Too Young For Him To Be Fertile? Is There A Fertility Booster?

The hen start laying eggs after 6 months of age but roosters can take few more time to become adult. Usually roosters start their job at the age of 8-10 months.
Hen don't allow the roosters to start their job until their size match with them. So, not only the rooster should be young for mating but also having good health and body size of hen.
There are are some multivitamins available at poultry stores which will boost up the fertility in roosters.