When my daughter was about 3 she began throwing up a couple times a week. As soon as she did she would be a little weak at first then totally fine. I knew this was not normal and my pediatrician had no ideas! Luckily because my nephew has some serious food allergies I know they can affect you in all different ways. I had her allergy test done and she ended up being allergic to a number of foods, including peanuts and corn. Corn is in so much! Think about syrup, ketchup, cereals, cornstarch, syrup ,flour seems to be in everything. Once we found this and I began limiting her foods with these ingredients the vomiting completely stopped. She is almost 6 now and alot of her allergies have gone away. She is still allergic to all nuts and the others are very mild. I would definetly have him tested. Talk to other moms with kids who have allergies you will find out so much. Good luck!
Dont know where you live I do know here in the US we have had a nasty stomach virus going around it takes a few days and they are okay ..make sure your child is hydrated and speaking to the doctor may be a good idea .