Since your son is suffering from diarrhoea
one will have to rule out bacterial infections by doing stool test
.if it comes positive for plenty of pus cells then he will need to take appropriate treatment.
If the stool test comes normal and child is otherwise alright without fever
then the second most common reason for such diarrhoea could be toddler's diarrhoea .
Toddlers diarrhoea can spontaneously resolve as the child grows or by modifying your son's diet like giving more fat,less fibre,less sweet juices and fluids .
I would advise you to get him examined by his doctor so that he can advise investigation accordingly.