What Causes Excessive Flatulence And Fecal Incontinence?

My son has CML. He is on Sprycel 100 mg po daily. He has been experiencing severe constipation for the last three weeks. He passes a small amount of fecal material and flactuance. His abdomen is very distended. He has not lost his appetite but feels his stomach is very slow to empty. We are getting a cbc, mbp and ua and gastrographin to check for blockage. If all good, what else coukd be causing this. We know chemo makes digeztive system slow, but he has been taking Sprycel for two years. He was diagnosed at tgheage of 21. He is doing really well other than this digestive issue. Any suggestions? He has good diet, fiber in diet, drinks only wTer, sometimes gatorade. He has taken stool softners. All this with very small results. Major change in his bowel habits these past three weeks. He is active as well. Any suggestions? Thank you, Worried Mom