Green snot symptoms and conditions

Hi, my 6 yo son started on the 4 mg singulair about three months ago after being hospitalized for three days due to a seizure. After many tests and massive iv antibiotics it was determined that his sinus infection (for which he was taking an antibiotic at the time) had irritated his brain and caused the seizure. He has struggled with sinus infections ever since age 1 and taken way too many antibiotics. After starting the singulair we noticed that the next time he got a cold his nasal passages stayed open and after blowing green mucus for about three weeks he was better without an antibiotic. We thought singulair was a God send. But out of the last nine weeks, he has had green snot coming out of his nose for six of them. He also frequently complains about mucus in his throat. But the thing that sent me searching on the internet was that now he is complaining of stomach pain. I feel like I'm between a rock and a hard place. He can't handle any more antibiotics, but I don't like what the singulair is doing to him either. I welcome any thoughts. BTW, we did have him allergy tested recently and he his highly allergic to dust, but nothing else.

Julie