it will take about 10 days to be eliminated from his system. be prepared to be amazed at the change in his behavior / personality. it is a shame that so many children are taking this drug, having side effects and nobody warns the parents. my son was suffering undetected side effects for 3 years.
My son also was on singulair from 2 1/2 - 7 1/2. I also thought he was experiencing terrible twos and threes and ....but after he had a seizure, I took him off singulair. It's been a year and a half. I still see some signs of lack of self control, but nothing to be so concerned about. The biggest problem has been his view of himself. Being on singulair so long, parts of his personality have been effected by the side effects. It makes him feel like he's wrong or "bad". But I continue to remind him that was the "old Ryan" on singulair, you can behave now and your a wonderful little boy.
Just hang in there. You will get your son back, but it will take time.