what is this nausea/diarrhea/bad tasting burps?

Boyfriend got sick a little over a week ago...had typical diarrhea and vomiting, lasted a couple of days...then it went away for a day, came back...it's been doing this every other day thing, some days consecutively for a little over a week now. He says it will start with burping, which tastes like cheerios (which is disgusting I realize), then he has diarrhea with some vomiting. He says is poo is really dark colored. He doesn't have any real abdominal pain, maybe a little soreness in the lower area. A few times he has described his vomit/bowel movements as having nothing to do with what he ate, like maybe it's all more mucousy like. But it just varies. What is up? Should he get to the doctor asap?