I have IBS-D, wee fun. Anywho, mine left for quite sometime and I'm trying to figure out if it's back. When it was really bad I was on Lotronex and Lomotil. I'm pretty sure Lotronex is off the market though- but it really worked for me. I've also done Bentyl and Donnatal.
I highly recommend www.ibsgroup.org
It is a wonderful self-help organization with an amazing message board and tons of info on IBS, treatments, foods, clinical trials, etc.
The best advice I can give is for him to do an elimination diet- this is important because what triggers him may not trigger others and vice versa. Caffeine, chocolate, and spicy foods do not trigger me, but they are a major trigger for some others.
With the elimination diet start off with clear liquids and move up with bland foods, adding one every few days or so. Start with water, tea, broth- advance to plain boiled potatoes, applesauce, jello, advance to plain toast and just keep going. This will help identify triggers. The BRAT diet is a good idea to start with- Bananas, Rice, Applesauce, Tea (herbal tea like chamomile is a good start, as Green Tea could potentially be a trigger).
Keeping a food, stress, symptom, and lifestyle journal is important. He should log what he eats and what happens and if anything was out of the ordinary or stressful that day. Also pay attention to stress symptoms like tightening jaw, grinding teeth, stiff neck, etc. Stress can be a huge trigger.
I had success with cognitive behavioral therapy, diet, and exercise, with medications like Lotronex to stablize things.
I can't remember if you mentioned medical testing, but colonoscopy, sigmoidoscopy, endoscopy, upper and lower GIs can all be important in eliminating something more serious. Don't forget about getting tested for parasites as well.