Hair fall and some blackish thick pimples like stuff on my kitten

Hey! I need help as how to treat my kitten the affected areas are mostly on his back ! I have consulted a vet , I have been very worried but the point is that the vet told me to apply some medicine it's kinda foamy then dry it ... It's a heavy task. It's the fleas getting worse but I just wanna know if someone else's cat or kitten has this kind of problem and if it is fixable or not.
One more thing since my kitten is Persian and because of this infection the hair on his face are falling and the hair on his neck and areas behind his ears are already fallen. Will the hair grow again?
I am worried a lot!

In these pictures the blackish spots aren't visible, these are the areas which were previously black but now that the treatment is going on they hav turned into as seen in the pictures. The blackish spots are on the arms etc, does anyone else's kitten has this?

Anyone help me plzzzzz

Has your kitty had fleas?  I know some kitties have an allergic reaction to fleas.  I have only seen an extreme allergy once and there was quite a bit of hair loss which did grow back.