If it's happening when he's excited or active, this is very likely caused by a collapsing trachea. It's VERY common in this breed. I would want to see the vet to confirm this. Most dogs with this condition do not need treatment (aside from anti-tussins) and live a very normal life.
Aside from that, heart conditions can also cause a cough with exertion so I would want to have the vet pay a little close attention to the heart.
This CAN become a habit, particularly if he has been doing it this long. When he starts to cough, be sure to distract him with a toy or even just affection so that he will stop. If it's a habit, this will help.
But I would definitely want to look further at tracheal issues and the heart.
Respiratory illnesses are also a common cause of coughs but if your dog has not been exposed, I would not jump to that first.
You can read more about robitussin and its use dogs, including dosage information and the precautions:
Let me know if you need more help.
Yes. Coughing can become a habit in some dogs. If you distract her, you might be able to break the habit, as opposed to letting it continue.
An excitable cough (one that presents with excitement) is often because of tracheal issues. If this is the case, your vet can prescribe anti-tussins to help. Some dogs require surgery to repair a severely collapsed trachea.
Your vet can listen to the heart to check for a murmur, which would be indicative of a cardiac issue. Worst case scenario, you can get a cardiac ultrasound to rule out heart problems.
Kennel cough is still a concern but time/antibiotics should clear that up for you, if that's truly the cause.