There could be several reasons why he is coughing:

1) allergies

2) irritated throat from poor dental health

3) a preexisting heart condition

4) Collapsing Trachea

5) Bronchitis, sinusitis, or upper respiratory infection

Allergies can be inhaled or airborne and can result in throat irritation and sinus issues as well as the skin. He may be having a flare up of these allergies and an antihistamine might help if he isn't already on one. Benedryl can be given at a dose of 1mg per pound up to three times daily to see if this brings him any relief.

Dogs with a lot of tartar on the teeth and with present gum infection will also usually have redness in the back of their throat or tracheitis. This occurs because a dog continues to swallow bacteria from their mouth. This too can result in damage to the heart, kidneys, and bladder aswell. He may need antibiotics to clear the infection and a dental cleaning.

Heart conditions or enlargement of the heart can cause coughing by actually pressing up on the trachea or even causing fluid to build up on the lungs. Monitor his gum color as they should remain pink. Excessive coughing, pale gums, panting, pacing, or weakness are signs that indicate a need for immediate attention. The veterinarian will need to listen to his heart and lungs and may possibly take an xray to evaluate the size of his heart and condition of his lungs.

Collapsing trachea is a very common condition in smaller breed dogs. This happens when there is a weakness in the trachea or wind pipe and it literally folds on itself making it difficult for the pet to expel air. Usually there is a characteristic honking sound associated with this condition. It can become very serious and life threatening so monitoring of his symptoms as above is very important.

It is also possibly that your boy has developed an upper respiratory infection or possibly a mild bronchitis. Antibiotics may be needed if infection is present or if he is running a fever. Robitussin can be used for coughs but it is contraindicated in dogs with potential heart conditions so I would prefer you request a dosage from your veterinarian since he/she is familiar with your pet.

Overall, I would recommend a veterinary examine because as you can see a cough could be something very mild or could indicate the presence of a very serious condition. It is especially important if his condition worsens or the cough does not stop to have him evaluated. Waiting a day or two as long as he acts okay would be fine just to see if it is related to a minor irritation, but any more than that and he should be seen.

Thank you

Theresa