I am very sorry you are going through this. I won't go into detail about my son's past, but it is extensive and involves an unknown cause multi-organ failure, GI issues and surgeries, extended PICU stay when he was six months old. Here are some things I have learned from his medical journey, communicating with healthcare providers and being a parent when answers are non-existent. I recommend asking a lot of direct, specific questions to your healthcare providers. Take some time to formulate what exactly you want to know, write questions down, ask respectfully and take notes. You are your child's best advocate, and physicians will/should respect your involvement in your child's care.
For example, in your situation I might say something like, "It appears that we are just waiting to see if my son's GI issues resolve on their own. Is this correct? If so, how long are you willing to wait until further testing/diagnostic procedures, etc. are warranted?" "Are you concerned about the cause of this vomiting? If not currently concerned, at what point (what new symptoms) would you be concerned?" "What are some of your theories about the possible causes of my son's vomiting?" "Will you please give me a big picture explanation of my son's situation with his GI issues and how these could relate to his known developmental delays, seizures, etc.?" "I understand and appreciate that different specialists have tried to figure out the cause of my son's vomiting and have not found any answers. However, I would like more education on what is going on in his situation." Sometimes having greater knowledge and understanding of what they have ruled out is helpful when there is no known cause. (If you feel like education is helpful to you and not too overwhelming).
Also, if you feel you have exhausted all resources at that hospital, you can always seek a second opinion from a different facility. My son was seen at Children's Hospital Colorado which is ranked #1 in the nation for gastroenterology. However, I would first exhaust all resources at the hospital you are currently being treated at. I know how frustrating it is to not have answers (every department was involved in my son's PICU stay and none could find any answers for the cause of his multi-organ failure) but it is good to show respect for the healthcare providers, ask direct questions with definite timelines attached (to give you a better idea what they are thinking) and educate yourself as much as possible. Best wishes and try to take care of yourself too…don't be afraid to ask for friends and family to help you with meals, etc. Living in a hospital is exhausting and incredibly stressful when your child is suffering. Wishing you strength!