Good Morning and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
I am so sorry to hear that Prince is not feeling well...it appears to me that he is suffering from some form of digestive disturbance caused by either stress, food change or something odd he may have ingested. The Promax is good but does not have anti emetic properties.
Try this treatment for now along with the Promax that you have been given:
Fast Prince for 12-24 hrs which he is doing himself to allow the digestive tract to calm down and heal.
Give Prince Pepcid AC or Famotidine at 0.5 mg per pound every 8 hrs. That can be purchased over the counter at the pharmacy.
Give only small amounts of water during the fast or ice cubes
After the fast, feed rice and boiled chicken (2/1)
If the vomiting has resolved, then wean Prince back onto regular food. If the vomiting and diarrhea persists especially during the fast, take Prince back to your vet for an exam and lab work to determine the cause for this. This is extremely important since this can cause dehydration or other more serious concerns such as an intestinal obstruction or infection
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, XXXXX XXXXX