Kennel cough usually gets worse fast, how long has he been doing it? The vaccine isn't very effective, much like the flu vaccine for us so dogs can still get it anyways, it's why a lot of people and vets don't bother with it anymore. Coughing and hacking with low energy would make me concerned for heart issues but if he runs around without issue at the dog park probably not. It could be a trachea issue, sometimes dogs have it where they partially collapse or close up and that can cause coughing. Either way I would get him in soon, if it's a illness you don't want it to turn into pneumonia.