skin problems

hi, my little old man *rosco* is 12lb.s, and will be 14 yrs...april, he usually goes once a month, for shot for his skin allergies, i need, a liquid vitamin, too put in his food,for skin...and a good corizone spray (i only see 5%)his skin is horrible, dry brittle,hair loss, all down his back his tail,is so hard and dry, i used shampoo conditoner, and then have even tryed skin so soft, neosporin, on the inflammed spots, and he even seems too chew and gnaw, continuosly, if i am real sick, and in bed..i am disabled...i know hes old, and was never neutered, but i feel so bad, i need a really good mist spray, too apply during the day, i have looked, but i kind of wanted a 10% cortizone spray...i need your opinion...thank you...he also has arthritis, so a good multivitamin also..what products???? i was looking for a calming vitamin or herb, when he chews, but i did not like what i read, i dont want too hurt him with kindness, thank you, for taking the time, deby, and have a nice day........