What can I do to stop the digestive problems with Singulair?

My 9 year old son was put on Singulair about 8 months ago. He started randomly complaining of stomach pain and not wanting to eat, but we wrote it off as an "ages and stages" thing. Then, 2 months ago, the doctor doubled the dosage from 5 mg to 10 mg per day. His stomach problems intensified. He now has chronic diarrhea, weight loss (15% of his body weight!), and poor appetite. We took him aff the medicine about 2 weeks ago, but the problems aren't correcting. Does anyone know what will work to stop the vicious circle? The doctor wants to do upper and lower endoscopies, but that seems unnecessary for a drug reaction ...