What should I do to help him?

My almost five year old started “throwing up” in his mouth about 9 months ago. A little back story of what happened he started telling me he was throwing up in his mouth, I thought he was kidding. Then we started smelling his breathe and it was horrendous. We literally had to brush his teeth multiple times a day. He went for his routine 6 month dental visit and between his last appt to that one in 6 months he went from ZERO TO FIVE cavities. Immediately I thought about the throwing up. So we took him to dr they put him on Pepcid and it did nothing. We tried it for 3 months, same issue kept happening. So now we have been trying nexium for 3 months. Still hasn’t helped. What is the next steps? He is constantly “throwing up” in his mouth and I feel so helpless.