My son went on Singulair at 3 and did have severe side effects. "Adverse Reaction" is how it was put. He didn't sleep for 3 days. He was on the floor in a teantrum or running around - kicking, screaming, hitting, hysterical for 3 solid days, until it all worked out of his system and he recovered. I know that is an unusual and rare response.Eventually he went on Zyrtec, and today at almost 5 takes nothing!
Like Barb says, changing what the body is exposed to makes a world of difference. Over about a 3 month period we 'detoxed' our home and read everything we could, talked to people who got the chemicals out of their homes - No bleach,no chemicals, no artificially fragranced products, no body care products with artificial fragrance, colors, parabens, etc. We read up on the alternatives and now clean "grean". It actually is cheaper over all so the few products we have to buy "green" that we can't just use vinegar/ essential oil blends for, such as dishwasher soap, and fabric softener, are not outside the budget. Plus, my son is now OFF Zyrtec, and me and my husband are off our allergy meds. THAT alone is worth it!
Laundry, air fresheners & candles, and body care products seem to affect the biggest changes.
Also, we used NAET which sounds weird when you research it, but it WORKS. Safe, natural, non-invasive. The chiropractor helped a lot too.
Best wishes with your little guy and the new one!