It can take 3 months or more for a full coat to grow back in. A premium diet is key to a good coat – I recommend you go grain-free too. Most dogs don’t digest grains well, and grains can impact their immune system enough that they will develop allergies (usually allergies don’t appear until the dog is 2 years old or more, but why not take measures now to help keep your dog in tip-top condition?).
Go to a good pet store (not a grocery store or big box store) and ask to be shown to the grain-free dog food. Then start reading the labels – You want a food that lists real meat as the first 2 or 3 ingredients. Real meat is easy to spot – it will say elk, bison, turkey, pork, lamb, salmon meal (meal is the entire animal ground up and dried), whitefish meal, etc. Avoid beef and chicken – many dogs are reactive to those two proteins.
Do NOT buy any food that contains ‘by products’ – this is an indication that you will pay a premium price for a poor quality product.
Read the ingredients of treats too and use ones that are grain-free.
The one dog not being fixed is definitely the reason for the aggression. Get the 1 dog fixed ASAP then attempt reintroduction this may mean contacting an animal behaviorist for help.
Dr. Dawn is correct – you must get the second dog neutered, and even then you need to keep these dogs apart (so they can’t even see each other!) for several weeks afterward. This gives time for the testosterone to work its way out of the new neutered dog’s system.
Then, get the help of a professional behaviorist to reintroduce these two dogs – don’t attempt it yourself because they now have a history of aggression and will associate this aggression with each other. You need someone that can read the tiny signs a dog gives when they are uncomfortable with another dog. Even your own fear of something bad happening can trigger aggression between these two, so find a way to get help. If nothing else, contact your local SPCA and tell them your situation – and ask if they have someone that can help you, maybe a volunteer or a trainer that will help at a reduced cost. Explain that you are trying to avoid having to re-home one of the dogs and maybe they can help in some way.
Good luck – and don’t wait to get that boy neutered!