Dog is licking his underarm/paws! Watz wrong?

My shihtzu started to lick all his 4 legs. And his skin are red. On his underarm i noticed that some hairs are gone and theres like some flaking on d roots of his hairs. I remember i tried to pick them then some hairs went with it. On his thighs there are some black stuffs (like a birthmark) and he dont have them before. He had his food since he is a puppy and never had a problem with it.I tried to use an anti-flaking and anti-dandruff shampoo but it seems like the medicated shampoo is too strong for him. Then i switch him to an oatmeal shampoo and still lick. He licks bad last nite especially because he was left alone in my house cause we have to stay in d hospital. Is there any prevention i should do before i call a vet? Home remedies? Or good shampoo? Help!!