We had a dog with inflammatory bowel disease that also lost a lot of weight. They did need an intestinal biopsy to diagnose the problem--the symptoms are nearly identical to lymphoma and hard to distinguish without the biopsy. However, as I recall, the treatment for both was similar...or at least started out the same, with prednisone. If he's not up to, or you can't afford the biopsy, maybe discuss just treatment without knowing the cause. If it's lymphoma, it might buy you time--if it's something like IBD, he might actually rally!
It's a good thing you're waiting till the fungal tests come back before deciding, though. Fungal infections and steroids aren't a good combination.
I sure hope they can do something for Cody!!