My 7 Year Old Son Is Holding His Stool And

my 7 year old son is holding his stool and is very constipated...my dr advised to dissolve 16 caps of miralax powder into a 64 oz bottle of gatorade to clean him out...im need to know if this is safe for him? i gave him 3 caps full into a large glass of juice last nite but i believe he is still holding his stool