My younger son who's 10 has been having vomiting on and off for the past 3 years. Consulted doctors, done all tests and no pathological condition. But no one is able to find out the cause. How do I deal with this ?

Once he starts getting ready for school, the nausea starts and he starts to vomit either in the school van on the way to school or after he reaches school. The vomiting continues till evening if he has his lunch. He’s fine when he is at home during weekends and holidays. Done all tests and no pathological condition. But no one is able to find out the cause. Some say it's psychological. How do I deal with this ?