My 4 Year Old Son Has Had Bad Diarrhoea For

My 4 year old son has had bad diarrhoea for a few months now.he has had half a dozen blood tests and stool samples. All have said that he's fine apart from the last one which said low iron which he has a prescription for.this happened when he was 2-3 yrs old and caused major diarrhoea for him then and I don't want to upset his stomach any further.I have noticed tonight that he has passed a clear liquid from his bottom.should I be worried??