My 15 month old son has started chasing the dog...

We have a 5 year old terrier mix, who I love dearly, but she isn't the best with new people. When our son was born in December 2013 she left him alone or would sniff him and walk away. Lately though my son has decided he wants to "play" with her. He is tall enough to reach her when she is on her chair (which was her special spot) and she has a hard time escaping him. I am trying to teach my son to be gentle, but is is only 15 months and is driving her crazy. She has never snapped or anything, but she has started to warning growl. Does anyone have any suggestions? I don't want my son to think he has to avoid her at all costs, but I also don't want him to bother her. I am having a hard time finding a healthy balance. Help!