Hello,
Welcome to BPO, my name is Ang, and I have Sanity, a Great Dane, Hobo, a Chinook, and Araby an English Setter.
I can't help much with the hot spots, but with the underweight thing, what food are you feeding? You need to be feeding a high quality adult food with little to no fillers. If you're interested I can give you a list and I can also give you a spread sheet where you can grade the dog food you already have and find out how good it is. Also, how old is your pup? If they're a certain age, you can feed Satin balls, which puts weight on them, and is great for emaciated dogs.