I'm sorry, Amanda, but am glad you have a proper diagnosis and am hopefully that you'll be able to manage this and keep him stable for a long time to come.
Fwiw because obviously commenting on my dog's condition is purely anecdotal, ChiChi's autoimmune issue started in early 2009 at a few months after her 10th birthday. Up until then, she was an active and healthy dog with no issues whatsoever, and other than part of that year discovering and figuring out the treatment and handling of an inflammed joint and the ongoing dry eye issues, she lived very comfortably for an additional 6 years.
I think with Chi, a dysfunction of the immune system may have been a contributing factor since sister Dixie developed cancer later in life, and with that in mind, was also likely caused by overstimulation or overtaxing of the immune system with all of the vaccines they received over their lives, because 10 years isn't a very advanced age for small dogs like them.
ETA- sorry if that sounded like I was making this about my dogs because that wasn't my intention. I was trying to answer Heather's statement about development of autoimmune issues in older dogs.