Wow! When tube feeding a child (or adult) for medical reasons, it can take a long time to find resources that are truly helpful.
I stumbled across this e-magazine today and found article after article that was informative and well written. Check it out!
There is an entire section on GI issues and Nutrition. One of the interesting articles is in the REFLUX subheading and the article is about disorders that mimic reflux.
The author posts about symptoms, tests, as well as physiological categories like urology, respiratory, metabolic, etc. Definitely worth a look.