As you have previously been informed, there are different types, causes and even solutions to having hydro. I am one of the fortunate ones that was born with hydro and never knew until I was in my 20's. Apparently my body somehow compensated for the blockages in my vents. I only found out when I started having constant headaches (the most common symptom of hydro).
My suggestion would be that if your partner has no symptoms, don't worry about what may have happened when he was growing up. If he has issues that bother him, he could always go to a neurosurgeon for a consultation.
On another side, has your partner ever tried to gain access to his old records? It's always a possibility.
Either way, good luck with your quest.