Hello, please I need to ask about my 8 years old...

What it the tretment of gastroentritis ?
As you mentioned in your question that he has swan in a pool , which means it might be Polluted water and full of dirt so your son may caught a virus or bacteria , this explains his symptoms of vomitting and diarrhea , so after doing investigations doctor will prescrbe you a medication for vomitting to stop such as : Pramine either IV or Oral , also he will prescribe an antibiotic such as Flagyl Syrup 250 mg in a dose according to the weight of your child or Ciprofloxacin ,and you have to feed him light food and let him drink alot of water and warm drinks 
What are the investigations that should be done?
To diagnose the type of infection that causes gastroenteritis , you have to do the stool analysis test that ,ay show bacterial  or E.Histolytica or Gardia infection, and also CBC to see how WBC are elevated  
What are the symptoms of gastroenteritis ?
Vomitting 
Diarrhea 
generlized weakness 
Dizziness 
Abdominal Pain 
Advice :
you have to do the test and start the treatment