Montelukast - My son have asthma and by his doctor he is taking montelucast?

Merck makes Singulair. You can find those side effects listed for this drug. The patient information also lists many gastrointestinal symptoms and possibility of pancreatitis. It varies with every person but, elevated glucose levels can be associated with changes of mental status (outbursts, "hallucinations"); usually seen with very high glucose but, I have only seen this in adults. The pancreas, other parts of GI tract and kidneys also effects how glucose is managed by the body. If those systems are being effected by the Singulair, these combined body system side effects and a high carbohydrate diet's effects may lead to a person having elevated glucose readings.
Ask doctor for an A1-C test and about home morning glucose level checks. Both test kits are not that costly and are easyly done. The A1-C shows an average of what blood glucose levels have been over prior "3 months" timeframes.