To be perfectly honest this will not add anything unless his physician is not confident of the diagnosis and feels the need to exclude other pathology.
If this is not the case then the MRI will not add anything useful.
I assume that a MRI of the abdomen is to be done rather than just the stomach?
Lots of things could be seen - cancer in the liver , pancreas , bowel. Stones in the gallbladder. Adrenal tumors , kidney tumors/stones. Abscesses - really the list is very exhaustive!
The only one really would be a gastric outlet obstruction - but I assume he has had an upper GI scope already to exclude this?
Is he diabetic?
Has he ever had any gastric surgery?
Other than diabetes the most common causes of gastroparesis are..
Postviral syndromes, anorexia nervosa,surgery on the stomach or vagus nerve, medications, particularly anticholinergics and narcotics (drugs that slow contractions in the intestine), gastroesophageal reflux disease (rarely),smooth muscle disorders such as amyloidosis and sclerodermaNervous system diseases, including abdominal migraine and Parkinson's diseaseMetabolic disorders, including hypothyroidism
None of these will "show up" on a MRI scan - although you may see some features of amyloidosis.