You can tell your grandpa that while stress can make IBS symptoms worse it isn't the cause of IBS. The cause of IBS is still a bit of a mystery but there is good evidence that is caused by problem with the way the brain in your head and the brain in your intestines communicate with each other. You might print out the section in one of Heather's newsletters What exactly does it mean that IBS is a brain-gut dysfunction? These symptoms are not all in my head!" and give to your grandpa to read. There is lots of information in the IBS research library that you can show him too that shows IBS is not a result of stress.
The good news is the gut directed hypnotherapy is known to be very effective for helping teenagers and young adults manage IBS symptoms. It along with the dietary guidelines will likely help you alot. You can read about gut directed hypnotherapy here and you can ask questions about it on the "Hypnosis for IBS" board.
Ask lots of questions. There are many people here who can help you with ways to manage your IBS symptoms.
-------------------- STABLE: ♂, IBS-D 50+ years - Science of IBS