i presume he had a sperm test because he was trying to get his previous girlfried pregnant and failed? Are you in the sort of stage you could talk to him about his aspirations for the future? Does he want kids? If you do, and he dosent well then you have a problem!
if you do both want kids it would be sensible to get a bit more investigation
as an aside, when my sister was pregnant i found myself intensely jealous, and i have no idea where it came from. I dont like children, and didnt want any at the time, so it seemed so irrational!