I need to know a few details from you, before i can help you with your son's problem of vomiting
1) You said he vomits up to four times a week. On the days when he is vomiting how many times does he do so?
2) What does the vomitus contain - food he just ate, greenish coloured vomitus?
3) Is there an association of vomiting with ingestion of certain food items? Is there any correlation of vomiting with his meals?
4) Are there any associated symptoms with vomiting - abdominal pain/headache/giddiness/excessive irritability?
5) What is the weight of your child?
If you can get back to me with the answers to these questions, I will be very happy to help you out with your problem.