Im sorry you are going through this. I personally HATE this part of being a pet owner.
Your pup is 13 - and the meds your vet is giving you are the only that can be done without putting your pet through some serious treatments that it may or may not survive.
Live close to your pup, and talk about end of life possibilities with your vet.
I prefer to have a vet come to my home give my pup a nice cocktail of meds to relax him and then send him to the rainbow bridge in the comfort of my own home.
That your pup is 13 is a testament to you as a good pet owner.

If the cough is worse, he may need to change doses of medicines, especially to increase the dose of lasix. Can also ask about adding enalapril, which lowers BP to reduce preload on the heart. If those things don't work, then it may be time. I'm so sorry about your dog. He's a lucky boy to have had a good owner for so long.