Hello  dear
I evaluated your concern in depth .
* 
Dizziness & weakness is not related to 
flagyl , but it indicates underlying 
  inflammatory process going on in the body .
  reasons may be - low 
hemoglobin - low blood pressure - altered  
  electrolytes - 
vitamins , trace elements deficiency - low immunity -decrease   
  oral intake - inadequate absorption of nutrients by the intestines -             
  frequency of stools giving fluid loss & so many .
* Loose motions are only in relation with C-diff .
@ Maintain his hydration till you reach your doctor .
    I strongly recommend you to get the patient hospitalised , his condition
    is not stable , he require intensive management in the hospital set up .
Hope this will definitely help you to major extent .
Feel free to ask any further health related queries .
Thanks for using Health care magic .
Regards .