Our GI said to only use Imodium as a last resort... he'd prefer we try Cipro or Flagyl first (i.e. when traveling). If there's been a change in bowel habits he wants to know right away so DS can be monitored more closely in case its the start of a flare. So, in a nutshell, I'd suggest talking with your GI first.
For teenage stress DS sees a therapist. I think it helps? It just gives him an opportunity to talk to someone besides mom and dad. For tests, we take the practical approach - no procrastinating, plan ahead, etc. Good life habits to have anyway. It doesn't alway work and he's had to experience the consequences more than once before catching on, but... it does help cut down on test stress.