Hello, I am sorry to hear of this difficulty with the retainers. Having something on the roof of the mouth can take getting used to and stimulates salivary flow. There is a breaking in period. Rinsing with an astringent mouth wash may reduce the sensation. The extent of the retainer reaching to the back of the mouth should be adjusted as to reduce size and keep as thin as possible. There are other designs for retainers that do not sit on the roof of the mouth. An option may be one that sits on the teeth with clear acrylic. The clear retainer is often preferred by patients. Many orthodontists prefer the Hawley type on upper teeth especially if the upper jaw has been expanded. Please discuss this with your orthodontist. There may be some longer term options for a bonded retainer if the vomiting cannot be controlled. You do not want to have the teeth move after completing orthodontic treatment for lack of wearing a retainer. Thank you for your inquiry. I hope this information is helpful so your son can enjoy having his braces off and smile to show off his straight teeth.