It is unlikely that Singulair® (montelukast sodium) is responsible for worsening of your son's eczema. There are few reports but they suggest that Singulair® (montelukast sodium) may actually relieve eczema. In any event, we would advise you not to discontinue the Singulair® (montelukast sodium) until you are able to discuss it with his doctor.
You may want to seek a second opinion at a center that specializes in the diagnosis, treatment and research of eczema, such as the National Jewish Medical and Research Center in Denver, Colorado.