Suggest Treatment For Cough, Sneezing And Blocked Airways

My 11yr old went outside this afternoon first time in several weeks. He is a severe allergy sufferer and of course this affected his sinuses when he bathed and prepared for bed. He was given his singulair 5mg for first time this season around 9p. He was coughing, sneezing and both airways blocked at 10:30. I gave him 1.5 teaspoon of OTC triaminic cold and cough to to help with congestion. Now I m concerned I may have given too much med so close together and he is sleeping. Do I need to stay up and monitor his breathing and heart rate? Really can t go to sleep till I know if I should take him to er for too much medicine. He is not showing any sign of problems.