Sugar elimination from diet

Hi all new to forum and first time posting, really just to see if anyone have experience or tips on this. My 6 year old son who has autism has terrible problems with very loose bowels and hives when eating certain foods. Dairy has been eliminated from his diet for past three years but after paediatrician appointment today we have been advised to eliminate all sugars from his diet for three weeks to see how that goes and slowly introduce sugars back in. 

Any help or guidance really appreciated.