Cat Vomiting

I just adopted a cat with GI issues (diarrhea unless given prescription food - not vomiting) on Thursday. He threw up three times Friday morning but has been eating and drinking fine. He just threw up again this morning. I am waiting on the vet to return my call to get him an appointment, but they don't open until Monday. Is this just a stress reaction still? Hes been very affectionate and demanding for pets. He is over 7 years old.