Any chance he was exposed to another dog with kennel cough? He's eating and pooping ok, right? Just the coughing and decreased activity?

Have you ever heard of a "reverse sneeze?" It's sounds somewhat like a cough; more like he'd be trying to cough up something in his throat. That's all I can think of. If he continues to cough and act sluggish, I'd definitely give the vet a call and have him checked out

Oh, not sure the OFF! is a good idea for the pups as they might lick in off their fur...do ask your vet about Advantix. It's a monthly flea and tick treatment like Frontline, but also repels mosquitos!

Please feel better soon Major!!! Paws crossed for you cutie!