My understanding is that there are many reasons for dark circles and allergies is one of them. I'm assuming he has allergies, and that will contribute to your son's dark circles. And if he's tired all the time that will add to it. My daughter had the same thing when she was younger, and she too has asthma/allergies.
Get off Singulair and get on OCTACOSANOL. This stuff works better than Mercks ( montelukast sodium ) except it has no side effects and its quicker and cheaper. Then kiss your and your sons Asthma goodby. No more coughing at any time in the for eather of you.
bill103 ....at your service