Vomiting after eating is most likely caused by a minor illness or intestinal problem. With any sign, a physical problem may be all or part of the cause. Your child is much more likely to be physically ill than mentally ill.
When vomiting or laxatives are being used by your child to control his weight, he has a serious problem. He may have thought of it by himself or may have picked the idea up from a friend or something he read. The behavior is dangerous and is both physically and psychologically unhealthy.
The behavior is common enough to watch for it whether you suspect the problem or not. Watch for:
The behavior can cause permanent physical damage and even your child's death, if it goes to the extreme. Talk with him about the problem even if you do not think he has it. Talking about it will not give him the idea but will teach him about the dangers so he is aware of them when he hears about the behavior at school or somewhere else.
If you suspect your child is or may be involved in the behavior, talk with him. Also, closely watch him for a couple of weeks. If your concerns are not satisfied, get him to a children's eating disorder specialist. You and he cannot handle this problem without professional help. Even if the behavior seems to stop, keep an eye out. It is likely to start back up.