What did the vet suggest? My girl had HGE a couple of months ago and required hospitalization...it is very scary
My understanding is that all you can do is "supportive" care, which means keep him hydrated. I wouldn't worry too much about food unless your vet has indicated otherwise, but I would be popping him full of pedialyte as often as possible, and then keeping him warm and clean. Any sign of worsening symptoms I would get to the nearest vet hospital ASAP.
Prayers for a quick recovery.