If you don't mind my asking why are you having this treatment? Kennel cough is a self-limiting infection like the common cold. The vaccine is a 'modified live' so can often give symptoms in a previously healthy dog. Since it also mutates like crazy there's no guarantee your dog won't get another type anyway. Even worse, dogs given the vaccine can shed it for some time later- to other dogs.
As dogs age there's a lot of good science that suggests no vaccination is best. Please understand I'm not being judgemental with you as an owner- because I'm in the position of having to have my middle-aged dog vaccinated so if we have to (in an emergency) he can go into our local kennels. But I know it's bad for him and have tried to reason it out. They have their rules and so it goes...
Either your poor boy could be getting a dose of the infection each time- or could be reacting to carriers in the vaccine (aluminium for one). Unless you are in a position of being forced into it, I'd stop vaccination from now on.
