Lady's had HGE three times. Thank heavens you took him to the vet. They can go downhill in just a few hours and die if not treated quickly. Their blood thickens and clots up, plus they can go into cardiac arrest.
Did your vet say what his packed volume was? The first couple of times Lady's was borderline, but the last time it was really high. I nearly lost her.
Here is some information. Everyone should know what signs to look out for as toy breed dogs are prone to it.
Hemorrhagic Gastroenteritis (HGE)