Since you say his tummy is getting distended- its better that you take him to ER.
His hydration status needs to be assessed, his bowel sounds should be listened to and then treated accordingly.
If doctor feels his tummy is fine, he will give you medications for his vomiting
and will advise you how to maintain his hydration.
is severe, tummy is not fine, he might need to be hospitalized.
So its better to take him to ER before conclusions.
Wish your son a speedy recovery.