Black Smoke Persian Kitten with thinning hair/hairloss

I have a 12 wk. old Persian kitten, (Black Smoke) that was very tiny when I got him.. he was sickly.. had diarhea/coccidia.. quite often I took him to the vet finally got all that cleared up.. and he''s all fine with that,, playful,,etc.. now I see his hair starting to thin out,, hairloss.. on base of tail, back of feet.. underbelly some.. it is changing color too. he was all black,, and is slowly coming back in the light grey where its thinning or missing... Just wondering if this is how the smoke color comes in? is it just a coat change or illness? I am not sure.. Wondering if anyone else experienced this.. Even the spot where the vet shaved to do his bloodwork came in light grey.. unless its just the undercoat? I don't know. Thanks for any help.. and yes, I could take him to the vet.. and will if it becomes a major problem..He is very active and not acting sickly at all...

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