I hope he'll recover! That's scary stuff - I'm surprised your vet didn't have him hospitalized. Right now I'd be following strictly following veterinary advice to get him through this, even if it means temporarily putting him on a prescription diet until you can stabilize him. It's expensive, but sometimes it's worth it in the short-term until you can find a diet that fits your budget better and works for him. Especially with a potentially fatal condition like HGE.
As an aside, while plain, natural, unflavored/unsweetened yogurt is a good source of natural probiotics, I wouldn't be feeding any dairy to a dog with GI trouble. It's gentler on the stomach than some dairy products, but it still contains a good amount of lactose and can be hard on the digestion, given dogs are lactose intolerant to a greater or lesser degree.