Oh my gosh . . I just looked it up and it looks very scary and painful. It looks like your pup has a long road to recovery ahead of him.
I read that is was a secondary infection. I found some information that may help you understand and figure out the source of the initial infection. From what I understand from reading it is from Yeast and Mold , an animal can get it through an air born fungus or through a cut.
http://www.cfsph.iastate.edu/Factshe...otrichosis.pdf Fungal Disease (Sporotrichosis) of the Skin in Dogs | petMD Sporothrix schenckii - Wikipedia, the free encyclopedia
If he isn't already I would suggest if possible putting his cage next to your bed in order to give him some extra sense of relief knowing you are near him. You both may be able to some much needed sleep.