Will he eat rice? That first change we made in our diet, going with an elimination diet which was mostly rice at first really helped. Luckily for us, Isaac would eat his fill of rice, and we could put "safe" foods with the rice to help nutrition but also to systematically observe what reactions he had. For us, we found out soy and dairy affect him the most.
Probably what to suggest is to look into elimination diets and see what would work in your situation. Just googling it and starting in is good for learning about it, but with diarrhea, we generally wouldn't worry about flushing the system first with fiber--just get into something starchy and without common allergens. With kids it seems like response is quick, but see what happens in your case. Introduce allergen groups systematically and see what causes what. Keep a rigorous in/out journal.
It might help.
I'm a big pusher of the probiotics. Isaac seems to do better when he's on them and do worse when he's off them, with greater impact than anything else we've done as far as we can tell. A good sacchromyces boulardii capsule is available that you can hopefully introduce. There are other probiotic powders that dissolve and might bring results.
I also bet that if/when the poop starts to be more formed and the urges less urgent, your son'll be more motivated.
You can check out our blog for more details. To help keep an in/out journal, you can start a blog to go with that daily record, and you can also have an outlet for your feelings too. This whole experience, especially last fall and into the winter, was a really tough time for my wife and I, responding in our ways to Isaac's condition/treatment, but of course it also has bonded us together all the more at the same time. And you're on the right track! Involvement in a forum like this is also a great source of shared experience and others who can understand some of what's going on.