I checked the wikia and was a bit confused. Hiram died unexpectedly by being scared to death. I was going to try to get my next batch of kids to Uni so one could go paranormal and resurrect him. However, his older brother was in a bad mood, destroyed his urn and then cleaned it up. Can he still be resurrected or is his grave gone?

I've done several resurrections(zombies and perfect ones) but never have encountered this situation in TS2. I know that if you dial the reaper, the one doing the dialing needs only have met the sim that they're trying to bring back. Having not encountered a situation where there wasn't a gravestone or urn I'm not sure what repercussions there'd be.
The University guidebook says it has to be a "tombstone/urn that exists in the neighborhood or college town" to do a resurrection. I say get the one into paranormal and see if Hiram shows up later in the options for sims to resurrect.
Did you save the game after this happened? If not, and your last save was prior to the brother destroying the urn then you also have a chance to change Hiram's fate at that point.