a small older dog

Question
He has had a cough for about three wks. and its a dry cough and he has a hard time bring up the fleam.

Answer
Hi MaryAnn
A cough should always be checked out by the vet.  Especially in an older dog, as it can signify a problem with the heart.  The sooner you get on top of it the better results you can expect.  So, make an appointment just to get it checked out.  
Other possibilities can be some type of allergy to something.  But as he is elderly I really feel you need to get his heart looked at very soon.
With thanks,
Silva.
www.roxysworld.com.au