Dog Was Exposed to Parvo Won't Gain Weight?

I recently found out that the dog I adopted about 2 months ago was exposed to parvo as a puppy and the owner never took him into the vet to get any vaccinations. He was extremely skinny when I got him and I just thought that he was underfed by his previous owners so I've been trying everything I can to have him put on some weight (cheap dog food, puppy food, extremely stinkin expensive dog food, wet food, homemade food) everything.

I feed him 2X a day and he still won't put on any weight, but he poops like 5X a day. I made an apt to get him neutered in about a month. This was before I found out he was exposed to parvo so, should I make a sooner apt? and is there any other way to get him to put on weight? Also, is there any chance he may have the virus? Should I get him started on any vitamins? I Honesty know nothing about this.

By Nessie