Lady's had HGE three times!
Do you know what his packed volume was? That will determine how serious it is. Lady's first two cases were pretty mild, but the last time her packed volume was 82 and she was very, very ill.
Supportive care/IV is the key to survival with HGE. Thank heavens you got him to the vet quickly.
Here is a good article:
Hemorrhagic Gastroenteritis (HGE)