Friend, the symptoms that you are describing in your dog are secondary to medical conditions commonly seen in older dogs. So, you are right it is part of getting older. The weakness or splitting of the back legs is commonly seen when the patient has advanced arthritis of the back legs, spine or a combination of both. Him urinating in the house is very likely because he is reluctant to use his back legs to go outside; this is a common behavior with dogs that have arthritis.
The discharge in the nostril could be secondary to a localized infection, a foreign body, lumps, etc. to name a few causes.
In response to your question, I would strongly recommend a hands-on examination with a veterinarian. I know it must be difficult for you seeing him like this; but, we have really good prescription arthritis medication that will help him regain much of the function that he is losing in his back legs. In addition, your vet may need to start him on antibiotics for his nostril. In the meantime, you can begin to supplement him with Cosequin (glucosamine), you can buy this over the counter in any pet store, give at label dose. Flush and clean his nostril with warm water and a moist gauze or cloth. Additional treatments will be based on exam and test results by your veterinarian. If cost is a concern I would look into your local ASPCA or Shelter where they often offer veterinary services at a lower cost.
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