IBS/IBD

Hello all.
My son is currently dealing with either IBS or IBD. He struggles with going to school as he has to rush to the restroom often. His GI Dr. has prescribed Imodium AD as an everyday medication. Has any one heard of this? The ER Dr. was shocked to hear such thing! He'll be having a Colonoscopy soon, so I'm hoping we get some answers, as this has been going on for over a yr. Any advice on over the counter or even homeopathic meds I can buy him?

Thanks in advance.