Hello and thank you for asking your question on just answer. My name isXXXXX will try to help.
If he weighs 3-4 pounds, you can give him 1/8 teaspoon every 6 hours.
But you can NOT use the DM.
It has to be the plain Robitussin that contains only detromethorphan.
It does not work very well on collapsing trachea because the trachea collapses when there is a change in the air pressure in the trachea.
The robitussin works if he is coughing from a cold or allergies.
You can read more about collapsing trachea here :
Unless it is causing you, or him distress, I would not treat it.
Make sure you read that article I sent you (the link).
As he grows, the cartilage in his tracheal rings may get stronger and the it would stop.
If it is more then a few times a day, then you should see the vet.
He may have an underlying upper respiratory tract infection.
The vet can prescribe medications such as prednisone or codeine.
The only cure is surgery where metal plates are put in, around the tracheal rings, so they can not collapse. That is only done in severe cases.
Human medications will not help him if it collapsing trachea. So nothing from the drug store will work at this point.