The following link takes you to the introductory phase of a diet that has helped some people with Crohn's or Ulcerative Colitis. Depending upon whether it works, then you can decide whether you want to continue with the diet (SCD diet):
http://www.breakingtheviciouscycle.info/beginners_guide/the_intro_diet2.htm
Some people promote Ensure as a way to boost calories, but I hate to recommend it because it is filled with sugar, corn syrup, maltodextrin, soy oil, carrageenan and other ingredients that aren't really good for anybody. Some of the ingredients, like sugar, can make diarrhea worse.
The BRAT diet, (only on a short term basis - a few days or moments when things are at their worst because not much nutritional value) sometimes helps people with diarrhea, but I'm not sure it is good for Crohn's because of the grains included. It means bananas, rice, applesauce, toast.
I hope they diagnose him correctly, soon...as a parent, I know just how worrisome it can be.
Please keep in touch with this site with any other questions or if you just need to vent:)
Meanwhile, there is another web site, targeted more at parents: www.dragonpack.com When you get to this site, just click onto "IBD Support Community"