Immunodeficiency

After having to put my elderly cat down due to FIP, I adopted Freddie Mac, age 5 months, from the SPCA. He arrived with ringworm and earmites, both of which were successfully treated. But during the treatment, he developed demodectic mange, almost unheard of in cats. My vet said that perhaps Freddie's immune system was compromised by the two former conditions and that at his age, his immune system was not fully developed. Freddie was treated mainly with weekly shots of Ivermectin - about 3 - and the mange disappeared. Now, 6 weeks later, it has returned. In both cases, it has been all over his body, not localized. Now my vet diagnosed Freddie with immunodeficiency, caused either by genetics or a lack of colostrum as a newborn. He has been tested for FIV and FLV and both were negative. Here are my questions, which I hope someone can answer. Is this sufficient evidence for a diagnosis? Are there no blood tests to determine his condition? Should he be tested for other diseases that suppress the immune system? If he truly has immunodeficiency, which is so hard to bear after having just lost my older cat, what are some things I can do? He is strictly an inside cat and he is on daily doses of Interferon. I was in such shock when at the vet, I couldn't even think of these questions. When is a cat's immune system fully developed? He will be 9 mos. in 10 days. Any thoughts or suggestions will be appreciated.