So: what is his dosage, per his age and weight????
Because, like anything, if too much dosage is taken... it can then cause more observed side-effects... such as moodiness.
Dosage, is per age/weight. Not necessarily age.
Singulair, has a Kiddie version???
There are many OTC allergy meds. Which are basically, antihistamines.
There are: Claritin, Allegra, Singulair, Zyrtec. (I personally do not like Zyrtec, because it makes me tired and moody).
EVERY OTC allergy med... will affect a person differently. It is not a one-size-shoe, fits all... regarding any side-effects.
If you don't want to use Singulair, then don't.
Ask your Pediatrician, for another suggestion.
You don't 'have to' use Singulair.
Just as I don't have to use Zyrtec, just because my Doc may suggest it. Because it makes me feel real crappy.
I use, Claritin.
Now about his sinus infection: Yes at this stage of it antibiotics are needed. The sinus cavity, is a dark dank cavern, and simply looking in the nose a Doc can't even see all of it nor what is all in there.
My Hubby has problems with Sinus infections. He is just prone to it. Many people are this way. It is just him and his nose that he is prone to it.
Now (even my daughter at 3 years old could do this)... you can try a "Neti Pot." It is a nasal flush. There are many kinds. You could try that for him. But you need to do it per the directions. But for my Husband and daughter (when she is sick), this greatly helps, their Sinuses.
Or, you can try a saline nasal spray.