It's scary to see something new like that in your dog! Kennel cough really is very much like a cold in a person. I know there's a vaccine for it but I will not use it. What I do is boost my dog's immune system to try and help them be less affected by anything contagious <or any parasite> they might encounter. At the shelter where I volunteer I routinely work with kennel cough dogs but is true that we haven't a dog with it, in our house.
Writing from personal experience explaining just what I do, not giving medical advice - I do a lot of garlic to help boost my dog's immune systems - especially before and after medical procedures, or if I find out that they were exposed to something <like the dog flu> after the fact <I did not vaccinate for dog flu either, although my youngest dog did go to classes in an area with confirmed cases...that would be a good example of adding in immune boosting things like garlic>. I also do a lot of garlic in the Summer as I use no chemical flea or tick protection at all and we live in a rural/wooded area.
I use whole, raw organic garlic <no, it's not toxic in -CONTROLLED - doses appropriate for the size of your dog>. My girls are 65 and 75 pounds or so, and when I use the garlic I can do -up to - two <2> individual cloves per day, per dog. NOT a whole head of garlic, though it would take more than that amount to present toxicity in dogs the size of mine.
Also, my dogs are used to garlic; I would build up the amount specified with a novice <"un-garlicked"> dog in my home. I generally give them the garlic sections up to 4 times a week and then give them a break.
I cut up the one or two <1 or 2 individual> cloves or sections into fine little bits and dump on my dogs food.
I also have an immune-boosting mushroom complex that I originally got specifically because my big girl had Rabies-vaccine induced cancer; I still use it because additionally now we have the knee-injured dog with plate and other parts in her leg, and bone cancer is a problem in the breed, with the plates adding a slight concern into that mix.
I also have an herbal tincture that has immune-related herbs in it like echinacea. It's a liquid mixture formulated specifically for animals by a master herbalist.
Much of the immune system in people and other animals is in the gut. That consists of the "good bacteria". If that's out of balance that can lead to a host of other problems over time. So I also do probiotics for myself and for my dogs <though different types>. In both cases I use non-dairy probiotics as I can't have dairy and dogs generally shouldn't have dairy either.
You may see something about probiotics sprayed on dry food. That is useless. There's so little in there to begin with, and then by the time the bag is opened by the purchaser, more organisms will have died, and as it sits and gets re-exposed to light, heat, air etc, the probiotics that may be alive continue to degrade.
There are also specific essential oils you can diffuse to help your dog cope with the symptoms of respiratory issues. If you ever consider that, be mindful of quality and research the oils you have in mind.