Vitamin C is anti-viral, and I would slowly work up to a maximum dose your dog can handle. Being young, I would use sodium ascorbate (NOW brand has a powder), since the calcium ascorbate might throw off the calcium balance for a young growing dog, and regular vitamin C is too acidic.
You want to help build up the immune system, as this is a combination of a virus and an immune system that can't keep it in check.
There are some threads in the main dog section that you might be able to search through. On the other forums I've been on, folks have just supported the immune system and they resolved.