From what you say, its likely that your son is suffering from gastroenteritis- most commonly viral in this age group- and unlikely due to fall.
Do visit your Pediatrician
He might need medications like
in a dose of 15mg/kg/dose repeated every 6 hours for his fever.
syrup in a dose of 0.3mg/kg/dose repeated every 8 hrs for his vomiting.
Along with these, take care of his hydration by giving him plenty of fluids- water, soups, rice based porridge, soya milk, ORS etc.
Give him plenty of curd.
You may also give him probiotic
scahets (econorm etc) that will help in reducing the frequency and duration of diarrhea.
If he refuses to eat/ he is dull and lethargic/ frequency of urination is less/ has high grade fever/ you see blood in stools/ has severe abdominal pain
, then you must visit your Pediatrician at the earliest.
Hope my answer helps you.
Wish a speedy recovery for your son.