If semen or pre-ejaculate got onto your vulva, then you do have a potential pregnancy risk. You also have an STD risk.
Regarding his "sterility": it's very rare for a teenager or young man to have any disease or condition that would make him 100% sterile.
I don't mean to make you paranoid, but we've had cases on the board before of guys claiming to be "sterile" in order to avoid using condoms.
So unless you've actually spoken to his doctor or seen his test results, you might want to take his claims with a pinch of salt.
In any case, even if he was sterile, that still wouldn't prevent him from carrying or transmitting STDs.