Thanks for your query
Since your child is having high grade fever along with pain abdomen anf green stools so there is possibility that he is having bacterial infection.leg cramps
is due to electrolyte imbalance
I suggest you to get him examined by pediatrician
as he needs some tests like stool examination.CBC.and in case of infection a course of antibiotic is needed.
In the meantime give paracetamol
for fever,give oral rehydration solution after each loose stools.give coconut water.lemon water,curd.,probiotic .
Wishing your child good health