Asthma & Allergies

My 9 year old son takes Singulair to help with his asthma, along with Advair, Rhinocort and Zyrtec (he also has allergies). He started these meds in the beginning of 3rd grade, he had good grades (A's and B's) for the first quarter. Since then he's been struggling. He's in 4th grade now and is getting B's and C's. I'm not saying he's an A student, he's average, but does have the ability to do better. I often wonder if taking all of these medications has an effect on him. I was hoping to find something out or to see if someone else had this problem with a child. Thanks!