my boy is always vomiting

how old is he? Is it vomit or spit up?

The Brat diet stands for bananas, rice, apples, and toast. they can also have very bland foods like potatoes without anything on it. My son just got off of that diet like a month ago Its alwful by the end of the week he was so ready for regular food but when they have a viral infection (which is very common) you gotta do it, which could be why your son is throwing up!
I just copied some info on it from the brat diet website. I hope this is useful

Isostar, Gatorade and other sport antidehydrant drinks are great to replace lost fluid and minerals. Brat diet last for three days. It begans only with liquid and then gradualy solid food is introduced. Third day if symptoms have improved only solid food is eaten.
Procedure of brad diet:

day One - Brat diet
First your child should stop vomiting, then 24 hours you should give baby only clear liquid. Start by giveing your child a few sips of water every few minutes, then slowly introduce apple juice or tea with no sugar.

day Two - Brat diet
Second day introduce bland food. These include Bananas, Rice, Applesauce, dry Toast or saltine crackers.

day Three - Brat diet
Third day more solid food be added, like cooked vegetables, chicken, turkey

The natural solution for acute and chronic digestive problems.

List of food you should avoid during Brat diet:
All meet should be avoided during three day Brat diet, only turkey and chicken can be introduced on third day of diet. Avoid all three days of Brat diet: Milk products(milk, yogurt, cheese), Raw Vegetables(carrots, celery, corn, cabbage, lettuces, and cucumbers), most Fruits(citrus fruits like oranges, grapefruits, pineapples, and oranges as well as cherries, grapes, raisins), anything containing caffeine, soda, drinks of extreme temperaatures.